Sounds about right except I don't understand "the center from the donor would be approximately 1" larger...." I had measured about 1/4" diameter difference.
Also the back spacing will need to be same as the 19.5" wheel used. The 19.5" rim should be 6" wide and the old 18" was considerably narrower. It's all about the correct dual spacing for given rim width to prevent tires rubbing.

•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-