Motor wheels is the generic term applied to hub piloted that was used by GM dating back to the 30's. It is similar to the generic name of Budd wheels for stud piloted wheels and the Budd company actually manufactured very few of them.
Wheels originally made to fit your vehicle were probably 5 1/2 and no more than 6" wide. to accommodate no larger than 9.00x20 tires. 8.25x20 was the most common. Tubeless wheels would have been either 5.25 or 6x22.5 and designed for either 8 or 9 by 22.5 tires. 8.25 wide wheels is what I use on the steering axle of my semis with 275/80R22.5 tires. 8.25 wide wheels were never manufactured for your vehicle. It would take a custom manufactured wheel with a much more offset to clear your tie rod ends.
Tires are measured by bead diameter. I think you are measuring from the outer edge of the rim which tells you nothing. You have 20" wheels. The width is can only be determined by demounting the tires.
You will have to shop the used market for wheels. Either 5 1/2 or 6" wheels will work with your 8.25 tires. Tubeless wheels are very scarce and I would certainly not buy tire until you have the wheels in hand.
What is your motive anyway. Are you just trying to get away from the R-H5° wheels? Depending on where you are located, you may not even find anyone to change the Goodyear type wheels either, even if you find some. A lot of tire shops refuse to change any kind of tube required tire.
I have several two ton trucks that came with R-H5° wheels. I recently bought a whole parts truck just to get the wheels.
Here is a picture of the 3 piece wheels on the parts truck.
https://plus.google.com/photos/108197707187186692269/albums/6038904364790673153They are the even more rare older smaller hub wheels.
