Maybe this is the truck you drove
Check out that big 22 or 23,000lb Eaton rear axle. Also note the bumper design used on extra heavy duty models. A GMC 6500 is in a completely different weight range than a Chevy C 60, it would be closer to a Chevy C 80. Most Chevy and GMC trucks built with this cab design were in the lighter GVW ranges, extra heavy duty versions of these trucks ( 33,000 GVW) had much lower production numbers.