Paul,
I just recently swappd 63 sheetmetal to a 62 frame and vise versa. There are a few differences in mounting locations IE where the front of the cab mounts and where the bed mounts are.
The bigger difference was the front end. The 60-62 uses a tosion bar front end. Depending on what you are doing with the truck one might say the torsion bar is the easier one to modify. You can pull the bar out, rotate it, slide it back in and your truck is lower with out changing spring rates ect.
The 63 and up is on a coil spring between the lower A arm and frame pocket.. If you are planning on bagging the truck, you will more than likely want the 63..
60-62 also has an X frame for added support. I'm guessing for the torsion bar front.
Now most the rears you will come across are a coil set up on a truck arm. GMC I believe was leafs and optional coil but I think Chevy was coil standard.
I hope that helps a little.
Shaun
