it's definitely been modified from its factory days. The dash cluster appears to be more chevy style from 60-63. It's hard to say so far from the pictures you have, as some of the tell tale signs of GMC are gone if it is indeed one. Tailgates are interchangeable, but GMC's would say that, on it as well as the rest of the dash would be different. I'm guessing since the trim tag says GMC, that's what you got. It doesn't appear to be swapped, unless the cab was.
