Brad,

First tubeless tires came out in the late '50's, 22.5" rims with that bolt pattern on GM trucks are available, hard to find, look on farm trucks, need to make spacer sleeve to use the modern wheels with slightly larger hub mounting hole. Modern 9 x 22.5" tires fit OK.

Unfortunately you have a set of 20's, perhaps you can find someone who needs them; only other option is to keep searching for good 20" wheels.

While you are looking for 20's, keep an eye out for some 22.5's, might take a while for either. Sometimes one finds one or two mixed on a truck.

Ed

'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires.
'47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle.
'54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed.
'55 GMC 370 w/270, 420 4 speed, grain, dump bed truck from ND. Works OK.