You sure the diameter of the wheel center is 18" and not something like 17.5"? I guess I'm asking if you were to put that center in a new rim what would size will it fit in without any machine work?
Also to double check, the wheel center it's self has 8.75" dish in it from flange to rivet holes where it attaches to the rim?

8R19.5 tires sholdn't have any problems with rubbing or clerance, I didn't have any although my truck was not quite stock.

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-