Thanks so much for your help on this. I found the data plate on the driver's door frame but it's completely faded and I can't read anything on it. I'm guessing I have the 3800 since it's a dually.
Late to the party but on the data plate on the door post the printed material only shows that it is a Chevrolet rather than a GMC and the gross tonnage. That the max tonnage you are allowed to put on the truck if you haul loads and have to pay for tonnage capacity. The only other time it means anything is if you were to go across a scales and there was a question on weight capacity, In the past 52 years of owning my 48 no one has ever wanted to see the tonnage listed on that tag and over all of that time the tag has had probably six coats of paint sprayed over it and when I moved back to Washington in 1977 the Washington State Patrol officer who inspected it took his pocket knife, scraped the paint off the area that the stamped number is on and wrote it down on a note pad and that was that and that is the way it has been ever since. The only thing important on that tag is the stamped number and the original fasteners that attach it to the door post.
I have wished for years that someone would make decals that could be placed over the old original door post tags with cutouts for the fasteners and a cutout or clear panel for the stamped serial number. Then you wouldn't get your truck impounded if you get caught with a shiny new tag from Ebay or the truck trinket store you stamped yourself.
