That plate on the glove box lid, you speak of, is the "GMC Truck Parts Identification" Plate. This plate was installed on all GMC 450 series, & larger trucks. This plate informed you of the type engine, trans., rear axle, springs, brake type, wheel type, etc. of these trucks, as they rolled off the assembly line. So according to this plate, this truck in particular, came off the line, with a 426 GMC 6 cyl. gas engine.
