I think I remember 63 having the newer frame. I know 62 doesn't and that's why I had to do a little work to make the 62 jimmy bed fit on my 65 chevy. Could be wrong though.
To the OP:
Do like 66GMC says and swap in a newer 73-87 front crossmember. I plan on doing that with mine, and going with an explorer 8.8 in the rear. I would suggest leaving the trailing arms in the back. They're the hot ticket. I like them so much, I plan on retrofitting them to my 96 c2500 eventually.