Welcome to the Stovebolt
Your truck was not born with that rear...we'll need to see some pictures in order to help determine what you have.
The difference between the 20" and 22.5" rims is that the 20's are multi piece that use a tube type tire and the the 22.5 are one piece that use a tubeless tire. Lots of old farm trucks can be found running a mix of 20" and 22.5" as they were designed to be replacements for each other. The 6-lug 22.5's are a lot easier to find than the 10 lug.
Original wheel center hole would be 5.25"...you'll need to measure your rear hub to see what the hole size is since it's not stock.
Mike B
