Sounds a lot like my truck...
1951 Chevy cab, 1950 GMC front, probably 51 bed as it has a chevy tailgate...
Not quite sure how the 54 stuff would match up. You'd think that it would as GM was probably looking to keep costs down but there are such differences with the cabs. (at least the dash)
If i had to guess I would say the back end stuff (fenders to cab, hood springs, etc) would work and you'd take everything off the front end of the GMC to use.
If the project $ is good enough, heck you might have a foundation for another truck if you don't use it on this one.
Good luck either way!