[dana44]

Welcome to the forum, 72 Darts are pretty nice little cars, lots can be done with them. Since you neglected to say which engine she has in her right now, will assume it would be a slant six or a 318/360. If it is a slant six, the K member won't allow a V8 to bolt into it, but Schumaker (spelling) does a six to V8 motor mount conversion. If she already has a V8, great, makes life easier. As far as bigger engine goes, you can get all the way up to a 440 into the engine bay with headers, but this is highly not recommended due to the weight, they handle very poorly. The next and even better option is to do a 340 or 360 (even a 318) stroker kit. The V8 size handles much better and you can get (318) a 392 stroker kit, or the 360 408 stroker and have a lot better car to drive and handle and still have a very respectable 400-450hp with very little work. As far as the gas tank goes, there are plenty of places to get replacement tanks, replace the fuel line at the same time and save the hassle later, next item is the rear end. If it is an 8.75, good, if it is the smaller (access cover on the rear), that would need to be beefed up a bit for breakage safety to the 8.75. Also, slant six transmission will not work with a V8, so that too will have to be changed.

 

[tazdevil]

Actually, your K frame is probably designed for a V8 in the 318-360 range, as is. I really recomend doing a fair amount of welding on the K frame to strengthen it, as they tend to poorly welded as stock. Yes, Schumacher has the right mounts, as well as other items, including excellent gasket kits for the firewall, and engine struts.
For engine choice, I would build a 340 clone car. Awsome power, great reputation for it's power and longevity. A new 340 block is available from Ma Mopar, as well as excellent heads and manifolds.I prefer the Mopar ignition system, complete. Put a 750CFM carbon it, and fly!

 

[peterjon1]

Mr. Norm (Grand Spaulding Dodge) used to put 440s in that body style, but in my opinion the 340 would be a better choice for a street driven car. I'm assuming you want to corner decently. The 440 is a heavy chunk of iron in a relatively light car.
Before you start your project, make sure the rear leaf spring mounts are solid. Depending upon your local climate, the area would rust out, and the spring mounts would end up in the trunk.