Surfer,
That number on your ID plate is the serial number. Some states used to title by engine number too. No VIN on these old trucks, hidden or otherwise.
When I registered mine in MN, the state added a couple of zeros or Os to the front of that number to make it conform with modern day standards. If you have a good title, it should be easy to bring it in and get it registered. If you have no title, I would get it titled etc before you do any work.
Some GMC guy/gal will come along soon and tell you about your truck.
Is it a 3/4 ton?
