The easiest trans to install would be a 200R4. It will need some upgrades to be reliable if your planning to beat on it. Make sure you use a good radiator, especially if A/C is in the plans. The exhaust can be a issue, depending on budget and desired results. For a mild driver, find some early A body dual exhaust manifolds. They are quiet, leak free, and fit with no cutting or hammering. The next option would be headers. Kooks and American Racing are the only 2 brands that I know of that fit inside the frame rail, all others have one tube that wraps around the frame. That limits turning radius, and most likely interferes with the heater box.
The 7.5 rearend probably won’t last long. If your rebuilding it, use a solid pinion spacer and shims instead of the crush sleeve. I have heard one of the issues of the 7.5 is losing pinion bearing preload. I would also consider a aluminum cover with bolts to preload on the carrier bearings. Other than that, I wouldn’t put any more money in that rearend. Buy the south side machine control arms that are luster dor sale on this forum, and find a early A body 8.5 10 bolt.