their should be a sticker on the drivers door or door jamb with axle capacities and year built info,also the vin number in the dashboard, read it thru the windshield can be decifered to get the model, see a dealer or insurane agent, from the pictures that you posted its a 1 ton gmc van chassis with dual rear wheels, the drivers door info will give you the axle weight capacities to properly size the brakes, and their should be an emissions/tune-up engine id tag under the hood probably on the radiator support.
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