The hood, fenders and grill are cosmetic differences...
The fenders are different because the GMC has holes for the parking lights, where on a Chevy the parking lights are in the grill. On a '54 or '55 1st where the parking lights are in the grill mustache its not an issue, but for the earlier AD trucks this is a significant difference.
The GMC grill design stands taller and therefore the hood is different to allow room for the grill surround. The holes for emblems and badging would differ too.
