Single wheels with 8 lugs?
Options are 16" if they fit.
17" as original
19.5" from some (not anywhere close to all) later model trucks.

Not sure about 19" wheels, do you mean to say 19.5"?
Don't know of 20" 8 lug wheels other than perhaps some fancy modern ones.

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-