the frames any exactly the same from 78-87, from 73 to 80 the ends were formed to bolt to the bumpers, in 81 they started using brackets that bolted to the frame and then to the bumper. also sometime in the early 80s the rear cab mount changed from a bracket (like the front cab mount) to a crossmember. my buddies 81 has the brackets and both my 83s have a crossmember. i believe the frames are the same shape and should swap with no problem.
a long bed frame is longer than a shortbed frame.
with the thicker frame, not all 3/4 ton frames are thicker. i have a 83 k10 and a 83 k20 and they have the same exact frame. most of the other k20 frames ive seen are thicker, but mine isn't. so just because its a k20 frame doesn't mean its any thicker than a half ton frame.
