Here's a link to info on 3 and 4 speed one piece transfercases. I always like referencing this article, cause it has a pic of me on a 1st place run on the obstacle course at the Indy Four Wheel and Offroad Jamboree. I was running a 350 SBC with stk 4 speed and tranfercase and a lockrite in the rear. It was just a trail beater.
T-Case Tech
I don't really have a recommendation on T/case or drive lines, it all depends on your needs and use. The link that pjohnson referenced looks interesting. My current driveline has evolved thru the yrs/decades for my needs and use of the vehicle. It's a street friendly trail rig. In my case, I always seem to find the next weak link. When I find it, I also try to figure out if I can improve on it, plus gain a mechanical advantage in doing so. As I mentioned, I ran a sbc/stk 4speed & tcase. Then a 1st gen lockrite locker. I ran that for 10yrs or more, then I installed 4.88 gearing, Detroit locker and a frt ARB. Later was the sm420/dana 18 t/case and 36"tires. I went Dana 18 case cause it was a Cast Iron case, and I had broken several FJ40 cases with sbc's and bbc's. The dana 18case had a lower 2.46:1 low range and they were basically free. The SM420 was $100, adapter $500. I broke a couple 18 cases too but they didn't split in 1/2 like a toy case. Finally, I adapted a dana 20 case where the 18 case was, to have center rear output. The jeep 18 and 20 cases had the same bolt pattern. I figured out how to adapt 2.46:1 gearing into the dana 20, and had a true twin stick t/case. Each stick operates the frt and rearend individually. I centered the 40's rear end/ 3rd member by using parts off a Mini truck rear. IMO, offset rear output t/cases suck for my needs. They re fine for moderate wheeling and street use. My street Cruiser has a sbc/NV4500 5 speed and a stk 40 t/case.
Keep in mind swapping to a chev motor wont fix your electrical problems and you will have fabrication, welding and will be buying a bunch of parts to make the swap. That's why I mentioned earlier, keeping the installed AMC engine and finding a J**p CJ T18 trans/dana 20 t/case, if you nave the correct bellhousing. Your rear is already centered, so there would be minimal alterations to get it to fit. The T18/dana20 case is a solid unit. You could center the stk 72 fj 40 rear using the mini truck rear for parts.