Dave, the center piece of the '54 gmc grille is the same as earlier ones but the grille surround pieces are different.
And as you found out the GMC "long motor" is longer. The GMC radiator sits further forward than a Chevy with a filler neck sticking out of the aft side.
Fumblin', by '91 there were few differnces except trim between the Chevy and GMC. And GM has used interchangable body parts on their cars since the early '40s at least.
And for the last several years some "American" cars and trucks and "Japanese" cars and trucks share bodies and come down the same assembly line.
But in the AD series of trucks there were plenty of differences. The GMC using a high oil pressure truck engine while the Chevy used a splash oiled car motor for most of the run being a major one. There were many other small differences such as clamps on the brake lines on GMC and clips on Chevy, thicker frame on GMC, higher amp generators and on and on.
