[joedoh]

i think you would need better than a 42" diameter tire (thats about what a modern 11R22.5 is in diameter) to make a 6.13 rear gear palatable. the gear calc says 2000 rpm with a 42" diameter tire would be ~40 mph. so you would definitely want a 5th gear, an overdrive, to get you to even 50mph.

but even if you can find a 45" tire, its gonna ride like a buckboard empty with all those springs. maybe go drive a modern dump truck for a feeling of what you will be using as a work truck, then imagine it much much much rougher and harder to steer (no power steering in 51). I drove an empty grain truck about 4 miles home once, it felt a little like the steering wheel was connected to a marshmallow that was connected to an elf that shouted instructions to another elf that was connected to the steering box, then more marshmallows in the tire rods and so on.

given that its been sitting so long, maybe think about a modern drivetrain AND chassis swap, like an older one ton dually. it will likely have a similar load capacity as the older truck and will drive a whole lot better.