What year is the donor?
Tranny doesn't need a thing, unless you want to play with a different stall speed convertor more suitable for towing. Gary's made some recommendations on good ones in the past. Your motor mounts will work, just need to move them up to the other holes about 4 inches ahead of the current ones (on the frame). Springs would be a little softer, but not noticeably. I wouldn't bother changing them if you aren't going up to beefier brakes. I went from 8 to six in my blazer and it didn't make even an inch difference in ride height. Don't forget the radiator shroud from the donor, and the fuel pump relay is different between the six and
8, so get the stuff off the firewall too. Look at the rear end, while you are at it. The 6 cylinder units rarely had posi traction or even good limited slip units.