I purchased a 1985 GMC Vandura 3500 vehicle. The seller told me it was not the original motor ( he cannot be found now ) I don't know if it was rebuilt, changed or what. I have the original specs but found the spark plugs are not what the specs say they should be. How can I determine what motor is in the vehicle ? If I have to look for numbers where do I look, and then what do I do with them. Thank you