I used to have a turbo 350 with dana 300 in my 80 CJ5, with a small block chevy. My rear drive shaft was about 11" long. I really did not have trouble with the short drive shaft in the mud. I did lots of mud when I lived in Flordia. I now live in Southwest Washington State. I elected to go with a Ford T-18 to help enjoy the trails local to me now. The total length of a Turbo 350, transfer case adapter and Dana 300 transfer case was 36". The total length of a Turbo 350, transfer case adapter and the Dana 20 would be about 32.5". There is the option of using a Turbo 350 with the AMC 304 V8 if you use a bellhousing adapter plate that is about 1/2" thick to go between the AMC 304 and Turbo 350. Then add a floor shifter and your an automatic CJ5. If you change your mind about the Turbo 350/Dana 20, it's really not a problem to sell, and convert back to a manual transmission. My Turbo 350 transmission with the Dana 300 adapter sold in less than an hour, @ 75% of my costs! The Dana 20 adapter to a Turbo 350 is not as popular as the Dana 300 adapter, but I doubt you would have to wait more than a week to sell out.
dave