The VIN or body serial number is gospel. For help there go to oldgmctrucks.com and they have a database of serial nos. A 1/2-ton GMC of that age will be a 100 series truck. It will have 6 lug bolts per wheel. 3/4-ton will be a 150 series and will have 8 lug bolts. Other differences, too but these are the most obvious. One cool thing about the GMC's that Chevy did not have at that time was a longbed 1/2-ton. Good luck.
