I agree with Stu that 20" tires on your old rims are a good choice for a truck that old.

Or, if you're determined to run newer wheels I suggest you look into some 19.5" 10 lug wheels from a GM P30 chassis. I can't say how they will fit, or if they will even fit over your rear drums, but I can guess that they will need a bushing in the center hole (hub piloted). Otherwise they should be the same 10 lug on 7.25" circle I think you already have.
Not saying they are a perfect solution, but might be a compromise, and look at some 8R19.5 tires.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-