Check the driver door jamb, for the ID plate, & number. Also looking at the pictures it kinda looks like it has the larger front fenders, that a 400, 450, or 470 series GMC would have. In this case there should be a "GMC Truck Parts Identification " tag, on the back of the glove box lid, that will have all the trucks particulars ie: eng. size, trans. type, axle type, etc. Count the number of lugs, on the wheels. If there's more than 5, on the front, it is definitely one of the heavier rated trucks. The heavier trucks also came in the longer wheelbases.