Just went on the hunt a couple weeks ago for "the" 700r4 transmission. First thing I learned was, everyone has an opinion as to what is best. Nearly all of them will work, but there are some which are "ideal". What I learned was, the '90-'91 heavy half-ton trucks, and some cars, have this transmission (see below picture). Ideal transmission will have a "K" on the side of the case as you see, and the additional cooling passage. If you look at the photo, you'll see the cooling passage as a rainbow at the cooling lines. Look at the input/return lines and see the "raised passage" that goes down toward the rear. Also, the ideal transmission will come from a V8 vehicle, and you can use a 2wd or 4wd. If you get a 4wd transmission, you'll need the longer shaft and a tailshaft housing. If you get a trans out of a V6 truck, that is fine, just rebuild it with all the V8 internals for a "stronger" build.
This not my transmission by the way, just a picture I borrowed from the net. I did manage to get 2, K-case transmissions, but neither have the additional cooling passage. Learned about the cooling passage after I got them, my mistake, but they'll work.