GMC's usually have a metal tag inside the cab somewhere, either inside a glove box door, or maybe riveted to a kick panel, that lists major components such as engine, transmission, front and rear axles, etc. with weight ratings, model numbers, and other basic info, along with the VIN number or other identification. My 1961 GMC/American LaFrance fire truck had a tag like that which identified the engine as a 401 V6, and the 5 speed Clark transmission, plus front and rear axle weight ratings.
Jerry
