ATTN. Jeepers!! Need help!

[Digger 5]

How about a TH400 that was used with the Quadratrac? It had the same Texas bolt pattern that the D20 t-case used and it should be easy to find. This is probably your least expensive option.
Stretch will be req'd to make a decent d-shaft. The 20 will be a better option than the 205 and if he decides to wheel it later a 3.15 low gear kit is available.

 

[TreeClimber]

How about a TH400 that was used with the Quadratrac? It had the same Texas bolt pattern that the D20 t-case used and it should be easy to find. This is probably your least expensive option.
Stretch will be req'd to make a decent d-shaft. The 20 will be a better option than the 205 and if he decides to wheel it later a 3.15 low gear kit is available.

AMC TH400s have the same output as a GM TH400, with the exception that the clocking is off about 7*. The AMC Dana 20 can be bolted directly behind the tranny, but due to clearance issues requires an extremely skinny front driveshaft.

(edit to clarify) The AMC adapter behind the tranny does have the Texas bolt pattern, but the AMC tranny itself has the 6 round holes standard to any TH400. The AMC TH400 is unique also in that it has a course splined output shaft which is much longer than that of a GM output shaft.

To convert to a D300 is prefered, but then again you spend $$ to build one.

I went with an AMC TH400 and Atlas combo myself.

 

[High-Five]

Molesting a CJ5? Isn't that what you're supposed to do?
I have a CJ5 with a small block/TH350/D300 setup they do fit and as stated previous the driveshaft is very short.
Stretching the door openings out isn't a bad idea, Jeep did it themselves with the post CJ5 era Jeeps due to the fact they suck to get in and out of!
Sounds like you're on the right path and also as stated doors and tops will have to be custom made not a showstopper by any means.

Leaving the tank under is a great idea, mine currently has a cell in the tiny little cargo area and it takes up 1/3 of it..factory placed tank going back in this winter.

Have fun, put up a build thread so we can see your trickery in action!