Stick with or upgrade to 8R19.5 tires, they are nice on a 1 ton (my preferred tire size for a 1 ton). 20" if you could even find or make wheels would be a step backward.

I bought a pair of 8R19.5 tires from these folks http://www.ricksontruck.com/ had the best price I could find at the time by far. They can't be all that far from you.
Most any normal tire store can get them too. Commonly used on motor homes and step vans.

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-