The vin with an S in it should have been born with a 371 Olds engine that GMC called a 370 with a New Process 541 transmission behind it and an 18,000 Eaton 2 speed. It is a sister to my first semi that I drove over 50 years ago. However, it may have a Pontiac engine in it now because a Pontiac would have been original to a 400 to 500 series. A 550 would have the Olds engine. Fenders from a 400, 500 or 600 series will fit it. The cab is the same as a pickup. I have several of them in my collection now. The album also has a picture of my original one.

https://plus.google.com/u/0/1126369...1091222818&oid=112636930370126476644