Just wanted to thank everyone for their input!
My dad is now the proud owner of an L99/6L80E we picked up from a 2010 Camaro. Originally he didn't want to worry about cutting the floor, and so we were sticking with a 4L60E, but when we found this engine he decided it would be OK to cut, and I could just weld up anything he might need.
In the meantime, all the searching, reading, researching, etc has led me to decide that I should put an LS engine in my 1989 Grand Wagoneer.
So I just ordered a nice LY5/4L60E from an '08 Silverado!