Since it is a GMC it won't have a model number 3100, 3600, 3800, etc. like a Chevy has. A 1/2-ton GMC of that era will be a 100, 3/4-ton 150, 1-ton 250, etc. The long bed will measure approx. 7'-3" and the short bed 6'-6". The number of wheel studs is the best indicator that's easy to see. Good luck.
