3/4 ton has a longer wheelbase and bed. The frame has a thicker web, I don't remember if the channel size is larger. There are some who will take a 3/4 ton frame and chop it to the 1/2 ton length, then put in a 1/2 ton IFS and rear end, but it's a lot of work.I'd use the 1/2 ton frame, put in an IFS and a 9" Ford differential. Use whichever cab is in better shape, rust repair takes a lot of work and time.