Any ideas, thoughts, brickbats, etc. here? I'm mostly concerned about oil pan and possibly tranny clearance issues; everything else i.e. motor mounts, exhaust, fuel system, computer etc. should be (relatively) easy to sort out.
I'm thinking motor mounts might NOT be so easy to sort out. If you're unlucky the K-member mounts will be too close together and you'll have a to cut them off and do a ton of fabrication, if you're lucky, the k-member mounts will be far enough apart and you'll just have to fabricate an adapter between the two, which is far easier said then done. Whose to say the trans mount might NOT be way off either and you'd have to relocate the crossover piece for the trans mount.
Oil pan, look at moroso's catalogue, maybe they have alternate oil pans for the 3.8L, as well, many aftermarket off-road/lift places for Jeep's might have alternate oil pans to clear aftermarket lifts/steering equipment. That way you can avoid having to fabricate you're own oil pan, or at least a moroso pan might be easier to cut up and reweld than the OEM diecast aluminum pan.
If you can't find drawings of the motor/trans comination with dimensions, maybe you could find a Jeep JK that you could crawl around and measure yourself. I'd measure everything from the engine/trans mounts as the reference, i.e. where is the back of the engine from the engine mount, top/bottom of the engine motor mount. Beginning and end of oil pan sump from the motor mounts, etc. The top of the bellhousing from the trans mount, the top of the trans body from the trans mount, the longitidunal distance of the bellhousing (from engine to trans body) from the motor mounts.
Does the Jeep trans have a slip yoke for the driveshaft? Or you would have to adapt the Jeep driveshaft to the 60's Dart, those aluminum Driveshafts are NOT exactly cheap or easy to cut and reweld, let alone stick an extender in them.
At the same time, if the only problem you have is the trans won't fit the trans tunnel of the 60's Dart, if you're able to do this project, than cutting out the existing trans tunnel and rolling and welding in new sheet metal to expand the trans tunnel should be easy for you as well.