It's not a 302. GMC had 2 different inline 6 cylinders, "smallblock" and "bigblock". The larger engines, I think were up to 500+ cid, were used in the 5 ton and up trucks. I remember seeing one a couple years ago at the Hoosier Swap meet. Thought it was a deisel, too. Then I saw the plugs and distributor. I'll check my resources to see if I can come up with some more detailed info on them. I do know that they were replaced in late '59/'60 (depending on model) with the 60 degree V-6, V-8, and V-12.