I have looked into this and the 20" wheel centers I measured were not a good fit in the 22.5" rims, I don't recall exactly what aspect was off but think it was the offset primarily and possibly diameter as well.
At the time I determined that to make/modify/assemble 22.5" wheels for these AD trucks you need the centers from the factory 22.5" wheels from 56-59. Or something different altogether, but not the original 20" wheel centers.

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-