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Here is an idea of what you want to purchase and what needs to be checked prior to purchase.
Pistons, as you mentioned, will need to have the block checked to determine whether or not she needs bored, thus oversized pistons. There are three basic levels of pistons, being cast aluminum, hypertuetic(might be spelled wrong), and forged. Cast for stock applications, hyper or forged, a little and much more expensive, but not jaw-dropping expensive, hyper gives a good balance of cost and durability to heat.
Rods, unless broken or bent, can be checked and resized to make sure they are straight and true, which is much cheaper than new ones, if you can actually find them new or aftermarket, new piston end bushings installed if needed (I believe they are).
Gasket set covers all the gaskets needed for the overhaul, and would cover everything except your 4bbl carb itself, it isn't offered for the slant six (ever), which is understandable.
Oil pump, you would want high volume if possible, and if not available, break a new one down and open up the flow passages, it improves flow and volume and isn't a real ton of work, always good practice to clean up the flow on a pump anyway.
Distributor. If you have points (because it hasn't been converted to electronic at this point), plenty of aftermarket companies out there to convert at a reasonable price, Pertronics has been pretty decent about making conversions, so make sure the bushings in the distributor housing are good and the shaft doesn't wobble.
Bearings. That's up to you, but rod and main bearings are relatively inexpensive actually, should get the tri-metal bearings for durability, also go by the name Clevite 77.
Pushrods. Check them, verify they are straight, tips aren't worn out, and no, they don't get ground, they simply get replaced if worn. Now rocker arms are a little bit different story, the end of the valves can indent or over time form a wear pattern. This can be gently sanded/ground out of them to fix that issue, and if not mistaken, the rockers are both adjustable and bronze bushed and can be replaced to improve stability and rocker arm geometry.
The camshaft itself, which is where the power of the 4barrel carb gets to do its work put to the test. It is a solid lift cam, with solid lifters and adjustable rockers, but you don't want to go too radical, which would be difficult to drive on the street. So, from the specs on your stock cam, which can be looked up or profile verified with a dial indicator and degree wheel (all cam grinding companies can verify what you have and what they can do), but basically, and increase in the duration of the camshaft by 12-15 degrees and an increase in lift of .050-.075 inch over stock would wake the engine up without killing its drivability. There are several aftermarket companies that make new cams with this type of profile, they used to call them RV cams (about a step above stock, maybe a hair more). Or you may have access to a cam grinding company that can help out on this one, including refacing the lifters.
Lastly, the head. Given porting is probably out, clean everything up, make sure things like rough castings in the ports are ground with a grinding stone, or my favorite tool is a carbide burr cutter, about the size of your index finger to the first knuckle. It has a rounded end and it good to keep corners from getting too tight. We can talk porting and touching up the combustion chamber later, this is getting pretty long, but will get you headed in the right direction to get things started.
And one other thing, the "rebuild kits" that are cheap, are just that, unless they give brand names, they are designed for quick flip out the door running to an unlucky buyer.
 

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Looking at the profile for the Erson cam, the TQ20M looks to be a very good profile. What transmission are you using with the engine?
 

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Stock converters are rated around 1200-1400rpm, and given this is a fairly healthy cam in the lift and even the duration category, you would probably do best with something in the 2000-2500rpm range to loosen it up a bit and be able to idle without dying as soon as you put her into gear.
 

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And by the way, what is the condition of the transmission itself? Quite often I have found that a good working high mileage automatic transmission tends to last a very short period of time once a good engine is built. If it hadn't been built within say 20,000 miles, it is a whole lot easier to throw new clutches and seals in it to ensure it can handle the extra power you are about to throw at it. At the same time, a shift kit is always a good thing for both the 904 and 727 automatic transmissions, and Mopars are extremely simple in this installment.
 

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Well, the head work and cam sounds good, .060 off the head may be a little much, even though it is possible, I wouldn't take that much off, .040 would be about the most I would do for safety concerns and cost for new pushrods, but it is your money. The cam seems good, pretty healthy but not overboard.
With old engines, one thing I have seen more times than not, is that a valve job of any sort and the raising of the compression on an engine with many miles tended to blow the rings out of an engine within a couple years. The least expensive and easiest thing I always do to prevent having to yank the engine a year or two later and start over is to simply do rings and bearings when doing head jobs. For some reason the older engines just do this, the newer engines with hardened cylinder walls don't do this well into 150,000 miles on the bottom end and rings, cast iron blocks from the early 80s and back with less than 75,000 miles don't do it, but more miles than that and they do to the tune of 80 percent of the time, so I see it as simple insurance for about $150 or so. I hate doing the same job twice in other words.
The 4bbl to 2bbl adapter is a waste as far as flow goes. Given the hole size of the 4bbl squeezing through the holes of a 2bbl does nothing but run rich when opening, looks cool, but black exhaust out the tailpipe doesn't make you go faster. If the intake is made of aluminum, then you could modify it by welding in a 4bbl plenum, then tune the 4bbl to be the proper fuel/air ratio through jetting. Unless you can do this, stick with a 2bbl carb.
 

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Well, the carb/intake setup is up to you, if you have or can get a 4bbl aluminum intake, the 500cfm carb shown is a nice carb, easy to tune and performs well. The setup picture you show is at least double the size that is capable to flow through the intake so it is a waste, so no performance or economy can be had with this setup. With that, if you have the 4bbl intake it will improve, or, if the intake is aluminum for slant six, aluminum is a lot easier to adapt (cut and TIG) a 4bbl plenum into the aluminum intake and allow her to breathe properly, the runners are large enough to handle 500cfm in this manner.
The porting, that is one place I know lots of power can be made. I always use carbide burr cutters instead of sandpaper rolls and stone cuttters, I like to use one shaped like the end of my little finger with a rounded end and straight sides, which gives a good rounding shape and doesn't gouge the surface. Matching the intake and exhaust gaskets to the gaskets, open the bowls to the inside of the valve seats, open the space between the port sides and valve guide bosses, and then get into the combustion chamber itself. Scribe the head gasket cylinder rings on the head surface. Where possible, open the valve pocket to the edge of this scribed ring, whether that is rounding the sharp edge to meet it or flat to it. The quench areas of the head (left and right of the valve pocket flat surface areas), round those edges, too. Really round them and open them up. You want the edges around the valves themselves (intake and exhaust) to be angled so that at no point the valve lift distance is constant or equal from the previous .050 lift, meaning the valve pocket is always opening to the lift of the valve (prevents stalling of the flow, both into the cylinder and out for the exhaust). You don't want any sharp edges inside the combustion chamber because they produce hot spots and make the flow and flame front angle off and not burn as quickly. Leave the carbide burr texture alone, do not polish it. The little chip cuts they make the fuel/air and flame from sticking to the surface over a very short period of time. I have several examples I have done in the past that when removed years later look like they were put on and run and torn down the same day and they are totally clean, both ports and combustion chambers, meaning they are burning cleaner than stock or polished surfaces, and they don't load up the way ported and polished heads do. Both torque and hp numbers are better on the bottom (and top) rpm range, so really good for the street applications.
 
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You are welcome. Show a picture of the combustion chamber and I can walk you though it, done it many a time, all the magic is in the combustion chamber design and improvement over stock. It's not that difficult, especially with cast iron heads, just go slow with the carbide burr cutter and an air powered die grinder, that's all.
 
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Yes, you can do that if saving a couple hundred dollars for parts and the future ability to make the full job a couple years down the road, rarely longer, then yes, you can do your work this way.
You have to drain and pull the radiator and heater hoses (so they are out of the way) and grille first, make room to pull the cam out the front. You then have to pull the accessories (alternator and whatever else there is, power steering?), then the fan blade and pulley, harmonic balancer (need a puller, and don't use a three jaw puller, use the plate and screw it into the balancer threaded holes so as not to damage the outer ring, they like to call them a hub puller or a large steering wheel puller), then the timing cover and timing chain (might want to replace this, double roller will last and have better accuracy and not too expensive), fuel pump (careful of gas leaks, so tie it up high somewhere out of the way), distributor, then valve cover and rocker arm assembly, pull the pushrods, should be a temperature sensor wire on the front of the head, unbolt the intake/exhaust together (it is bolted together under the carburetor, leave that alone, but pull it back and tie it to the left side of the car/hood hinge to give clearance), unbolt the head, and then pull the lifters. At this point the camshaft will come out.
When you get the head off, look at the valve colors. A good sealing intake valve will be very dark, even light black or dark grey, whereas good burning exhaust valves will be tan in color. If all the valves are equal colors intake and exhaust, you are in good shape, variations of the valves looking closer to each other in color indicates leaking valves as in little pits that have developed or even burned edges, which can usually be fixed with a valve job.
Be sure to break the camshaft/lifters in properly, they make break-in oil with ZDDP in it, a zinc additive that helps harden the camshaft to the new lifters, and if not done, the lobes of the camshaft tend to wipe off and eat the motor from metal shavings and make you start all over. Read up on that topic and mechanical camshaft break-in procedures, and keep us informed of your progress and findings, will help where we can.
 
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