Well it can be, but it sure is alot easier doing it with a high lift tranny jack on a lift. Ive done it both ways, and on your back is alot harder.
I didnt even know they made a conversion harness to go from the 60 to an 80, thats some very good news.
I have a 2wd 4L80E a 98 model, i might see about getting an LSx case for it and having it rebuilt to go in my truck, becasue i know no matter how well you take care of these trannys, and even on a stock truck its not a matter of if but when.
Om the old full size truck we had, a 97 with the V6 the truck never towed anything, had the fluid changed in a regular basis, and at 98K miles the tranny crapped out, and it was 1800 bucks to get it rebuilt, and after that i just didnt shift right to me, but we got rid of it a few months later.
Anyways, this is a great write up.