spartman,
Although the SM420 is a good transmission, I think it would be a step backward for your '80 pickup. The old 2 ton would be good project for someone that wanted a truck this large. The cab proper is the same for all the AD rigs so this might be of interest to someone...although it doesn't appear to be rust free.
Most of the value in this truck would be in the eye of the restorer/owner. If you have two projects ahead of this one, I would think your plate would be full. It's OK not to save every stray puppy you come across. If you actually have title to this truck now, put it in the For Sale forum and see what develops, or Craigslist in your area. There might be someone out there that is foaming at the mouth for a nice big Chevrolet 2 ton with hydraulic dump.
Stuart
