Hy beausejour, welcome, the model number you listed is for a GMC 1/2 ton pickup 114" wheelbase, 5000 lb. g.v.w., if it came with a V8 from the factory it would have been a Chevrolet 283, American GMCs came with Pontiac V8s from the factory up to series 500. Canadian Pontiac cars were also Chevrolet powered, so someone may have swapped in a "Canadian Pontiac" [Chevrolet] V8 at some point. I have a Chevrolet 283 in my shop that has valve covers on it which say Pontiac. Somewhere along the line someone has put Chevrolet fender spears on your truck, if they haven't swapped the entire fender. It can be a bit confusing the differences between Canadian and American models, I hope this explains some of what may have happened.
