I just did a sm465/ dana 300 swap in my CJ 5 (it allready had a 304). Now that it's pretty much "done" I'm happy with it but it was a ton of work. I'm thinking a t18 dana 20 that come mated together allready and mated to the motor you want to run will be much simpler.
I will say I was not a fan of the 3 speed in my jeep when I got it. I'm not sure where your from but if you deal with rocks I just don't see it being low enough. The 4 speed with granny gear is great, on "mild" trails I don't use low range and just drop it in granny when I need to crawl/ comepression breaking.
I hear you on a V8 power, my theory with amc v8's is if it's in there allready and works keep it, if you are going through the touble of swapping in a v8 into a CJ and are inevietably going to have to do some parts searching, possible rebuildin, figurin and fabermacatin you might as well do it with a cheaper/ more common motor. My personal thoughts is the GM 5.3 is "the cheap motor" to swap in, they can be had with under 100k for very reasonable, factory aluminum heads/ multi port fuel injection, last till over 200k, 300 horse power with nothing more than a computer tune.