I sold my CJ5, and bought a YJ, because I couldn't fit 37s in the rear without moving the axle back. Moving the axle back wouldn't work without moving the fuel tank. My 5 was body trimmed massively, and had about 3 inches of lift. I've seen 36s on a 5, but it depends on the true outside radius of the tire, not the stated 'size' of the tire.
You *might* be able to get 39s on it by getting a GenRight fuel tank, then massively trimming, and moving the axle back about 3 inches. However, it's also my lowly opinion that you'll need D60s front and rear if you do run 39s, and the both need to be high pinions to help with driveshaft angles. Probably need about 4 inches of lift total (as a minimum).
You're looking at a BUNCH of money. Front D60, high steer arms, lockers, hubs, Rear D60 (high pinion). Narrowed both front and rear so the tires 'kinda' stay under the fenders. $$$$$$. Then custom steering, custom radiator (to keep the V8 cool), custom hoses, custom wiring. Need some adapters to fit the Chevy into the CJ, adapt the Chevy to whatever tranny you decide, and adapt the tranny to the D300 (keep that, it's good). Custom lifts, Custom driveshafts.
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$
Keep it 'little', keep the I6, tranny and t-case. Put some one piece axles in the AMC 20, and wheel it the way it is. That's a sweeeeeet CJ. Leave it alone.
edit - and there's not enough room to move a motor forward enough to make a difference. You'll need that room for a bigger radiator and shroud. Might get an inch or two, but not enough. And if you did move it forward massively, you'll have interference issues with the oil pan and the front differential. Plus, you're 'ballance' of the jeep won't feel right. Too much weight on the front.
TreeClimber out.