Generic info packet from the GM Heritage Center archive:
https://www.gmheritagecenter.com/doc...olet-Truck.pdf
It specifically says "Chevrolet" but GMC would be the same.
Also - if you are interested you can send in $50 and your VIN and they can return to you the invoice for your exact truck, which would include the original selling dealership, RPO content and pricing. I've attached a sample below.
http://www.gmmediaarchive.com/?page=1
If there is a particular RPO you get stuck on - ask here. I have most of them memorized.
K