If you will re-read, I said "It has a round rear housing vs. a 'Texas' pattern."
Both have the same round front mating pattern but the rear bearing retainer patterns are different. The '80 is round but when in '81 the dial and fork style speedo connection was introduced the rear retainer pattern was changed to a 'Texas' style due to the bulk involved with the speedo gear mechanism.
Dan Pawlic, the guy did the cryo hardening for my D300 shafts, has swaped rear retainers by drilling the case. The 'Texas' bolt farther out (and farther in, I believe he told me, for the '80 retainer) since there is enough gasket seat area there for either.
That is the only modification you'd need to make to put the AA rear output on an '80 case BUT the later cases are easy to find ... so why bother?
I've seen piles of cases around here that have been robbed of their input and output shafts.
The big problem there lies in that you'd loose the precious 1"+ that the 'shortie' TC offers which can become critical in a short wheel based CJ-5.
I almost went with the 'shortie' in my rig but changed my mind once I realized that if I started having rear output breakage problems (the biggest short coming of the D300) I'd be screwed if I wanted to then go with an AA upgrade since I'd then have to change the rear drive shaft length and even rear axel pinion angle if I did it.