There are 2-piece 20" lock ring wheels that were used in the early '50's on Chevrolet/GMC trucks that have the correct lug bolt pattern, but have the wrong wheel center hole size (4.75") and you need a 5.25" center hole. Those older style rims are still serviceable/legal and could be put on a lath and have the hole enlarged...just another option.
FYI, All 20" rims are 2-piece (or more) and are tube type...22.5" rims are ALL one piece tubeless.
Like Grigg said, start looking at 1956-59 1-1/2 ton farm trucks, they had the 22.5" tubeless rims you need with your bolt pattern and center hole.
Mike B
