I started with a 304 in a CJ. Then I put a fresh 360 in that CJ, and now I am running a fresh 401 in a Wrangler. I would highly recomend the 360 if its in good shape or if you are going to rebuild one. The 304 was ok but the 360 was a major improvement and will cost the same to rebuild as the 304. Well worth the effort and you can find parts easier then the 401.
TH-350 is going to be expensive when you start adding adapters to bolt to an AMC. I put a 727 in mine out of a wagoneer and was able to bolt a DANA 300 to it with no adapter. A CJ-5 gets pretty short with an automatic especailly if you have to start adding adapters.
Dana 300 won't bolt up to a th-400. There is a stock adapter that is hard to find and not very strong for a Dana 20/th-400. I have a Quadratrac th-400 and was originally was going to use it, but I found out the after market adapter for a 300 or the 20 was over $500 and you pretty much had to rebuild the tranny to put the correct output shaft in the tranny. Thats when a found a guy with a 727 still in the factory crate for $500. It was a no brainer at that point. Bolted it to my 401 and I haven't had any trouble so far.
Not sure what trans he has in his '74 (3spd) but I do have a '86 258 in the shop that needs one cylinder. I wonder if his 3spd will bolt up to that motor. If not then he will have to rebuild my extra T-176 and find a D300.