The grille supports are the same, except the holes are drilled in different spots.
I know this since I too installed a '64 GMC grille on my '62 C10. If I remember correctly the GMC grille bolted up to 3 of the 4 bolt holes on either side, just the 4th hole needs to be drilled.
Also both grilles wire harnesses used the same plug on the end of the harness, so it's a plug in & you're set setup.
David