Production figures for Chevy trucks of the '60s are readily available, but GMC figures are not -- according to one of these stories: 1) they were either lost in a fire, or 2) they're in one of 25,000 boxes in a basement somewhere in GM-land, take your pick.
As a rule of thumb, however, you can take the Chevy figures and divide by 4 or 5 and get pretty close to GMC production figures.
You might try
www.searchtempest.com to find your dream truck. Good luck!