Taken directly out of the GMC Master Parts book;
Stamped: 83110
Part Number: 2223573
Rim Type: LBW
Size: 20 x 5.50s
Motor Wheel, 5 lug, on an 8" bolt circle, 6" hub size.
So it looks like you have most of the information. They are harder to find wheels. I doubt anyone manufactures new 5 lug rims that will fit. GMC gave these up in the early 50's for the 6 or 10 lug wheels. I havent seen any on a GMC newer then 51' but that doesnt mean they didnt use them a little later.
As mentioned above, you can swap for 6 or 10 lug hubs and then you have a greater choice of wheel options available.
Jeff
