8x20 or 9x20 is all there is to the old tire sizes. 9x20 is 9" wide and a 20" rim. I would look in wrecking yards for 6 hole Budd tubeless wheels. They are out there, used on a number of trucks. (not just Chevy). You can find them on moving trucks, some busses, etc. I would NOT use the Firestone RH-5 wheels if that is what you have. They are called Widow-makers for a reason and were sketchy 30-40 years ago! I don't think it is even legal for a shop to work on those anymore.
here is a link to tire sizes and inflation (1956 shop manual supplement)
http://chevy.oldcarmanualproject.com/shop/1956sctsm/56sctsm1006.html
