I can't answer the question about driveline length differences, but that's a minor expense.

I can say that I have a TF999 and D300 behind an AMC360, and it has performed great for me. I have 4.56 gears and lockers in both diffs, with 35" tires. Installed a CV joint in the front of the rear drive shaft.

It's been rock-crawling quite a bit during Kentucky trips, and I like the combination. The auto is a lot handier than a manual trans. I don't have quite the compression braking with the auto, but it's not bad in low range, 1st gear. I installed a ratchet shifter so it'd be easier to change tranny gears without having to look at a quadrant.

I'd make sure your brakes are up to snuff. I installed a Chevy 1/2-ton power brake system on my CJ.

1977 CJ-7, fiberglass body, AMC 360 w/ headers, DUI ignition, Edelbrock intake and Holley 4150 carb, TF999, Dana 300, 4.56 gears lockers, York air comp, 4" susp lift, 2" body lift, BFG 35" M-T tires, Megashifter, AGR pump & box, REP8000 winch.