[Hutch]

RockCreek
First welcome to the best truck board on the web.
The glove box data sheet would be the first place to start.
If it was ordered from Gm it will have a code that would point to that. It may also have codes that point to special ordered items.
These are commonly refered to as DSO Dealer Supplied Options.
Take a pic of the Glovebox data sheet and post it for the experts to decipher.
You can also date the axles with the code on the tubes.
Some folks have found build sheets in the springs under the seat and behind the fuel tanks, it might be worth a look.
I was in Alaska in the 70's when the pipeline was being built and saw a lot of heavy duty stuff that I have not seen since.
Back in the 70's dealers would do just about anything you wanted to sell a vehicle.