I found a 6 lug hub piloted Motor Wheel as used on some GMC's among other trucks, and oddly enough this one is a 22.5.

Some info:
6 lug hub centered "motor wheel" Firestone Accuride #26494 hub piloted 22.5 x 6.00" with 6 x 8.75" pattern and 6.25" center bore

I've seen one other that was to squished and misshapen to use and a friend of mine has seen one as well.

Pictures
http://rides.webshots.com/photo/2828307770080251109sBVRpI

So there is a small amount of hope that you GMC guys with Motor Wheels might find a set one day and could run 22.5" tubeless tires.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-