It would help if we knew what size truck you were asking about? The 1/2, 3/4 and 1 ton 55 - 57 are basically the same, the difference being the GMC has a flat area with a bolt hole right on the lip edge of the grille opening. This is where the GMC grille bolts to. If you want to use a GMC on a Chevy you need to beat the flat spot out to match the rest of the opening. Or if you use a Chevy on a GMC you need to make the flat spot (about 2" x 2"), not a big deal.
If we are talking BIG trucks, all bets are off! GMC has to many models to keep it all straight and I don't think any of them will interchange with Chevy.
Regards,
Mike
