Go to this page on oldGMCtrucks. Your serial number data plate should be on the front of the driver side door opening between the hinges on the front pillar. You will need serial number and it will be broken down to model, year, ect. Good information. This is only good for GMC. not Chevy. Also have to add that this information that comes from the serial number was correct when it left the factory some 70 odd years ago... does not take into account changes made since then.
http://www.oldgmctrucks.com/page0010.html
