If everything on the truck is right, it is a 51 one-ton, heavy duty. 51 had the turn down door handles with wing vents, only year for that combo. 51 also had the series number on the side below Chevrolet, 53 had only the numbers. Should also have an 80 mph speedo, went to 90 in 52.
Curious, what are the series number on the hood side?
The clue to the series is the wheels and lugs, as Curt pointed out. The duals mean the heavy duty, which has different brake drums from the lighter trucks. There is another member who recently got a one ton similar to yours. There is also someone from Montana or Idaho who went through the wheel issue last summer, and finally gave up on finding a modern wheel that would fit. IIRC this truck has 18" tires, which can be expensive to replace. Wheels/tires are the downfall to restoration on this truck.
The original owner, probably a farmer, could have bought this truck with the bed and lift installed. If he did, the usual procedure was for the dealership to either buy the cab/chassis from GM and have it delivered to a bed company for installation, then sent to the dealer. The other way was for the dealer to receive the cab/chassis and have the bed installed locally. Sometimes the farmer would install the bed themselves, but they usually did not get the lift option, which was a more complex installation due to drilling the frame.
The bed Darren used is from a 3/4 ton, shorter wheelbase truck. If your wheelbase is 137", you should be able to put a one ton bed on her, but the wheels will not fit. About the only way to make them fit would be to swap the differential with a later model, single wheel pickup. That would give you better road gears, but you would have different wheels and tires on the front axle.
