The 4000 series "tractor" could be fitted with many bed options. Vacuum or air brakes. 15 combinations of transmissions. Several different V6's. 305B and 305C (stamped on a pad, front of passenger side block). The 305A was used for 1960 1000-3500 series trucks. A Diesel and even a twin tandem V6 was offered in some GMC models!
Did you find the glove box plate?
V4008PN4726E
The V is Vacuum Hydraulic brakes (HydroVac), 40 series, 08 wheelbase, P Pontiac MI plant, N 60-61 year designation, 3725 th off the Pontiac Plant line (a GMC only plant) (Some plants started with 10,001 and some with 1001). E is D.O.T. GVW rating. A unique 2 year only hood, 2 year only round V6 emblem. Big Block GMC specific V6. Much torque, only 4000 max RPM, massive oiling and cooling designed passages and pumps. Good motor. For most people it's a boat anchor. You may or may not fall in love with it. I like mine because it's original. Otherwise I would not use it. I would use a SBC or BBC. I bought a running 305 V6 and a SM420 for $200.
Further info/options are by observation and the data plates. You may not need a manual if you are not doing anything technical. There are brochures and manuals on Ebay all the time. The 1000-3500 series manuals are what I have. Have see manual for 1000-5000.
I do not know about Big Trucks like this. I just like searching. It gives me surface knowledge. I do know all about the 60-66 GMC 1/2 and 3/4 Ton trucks.
