Do the math, 38" - (your tire size) all / 2

If you're swapping axles as well then it can be more complicated to figure depending on springs and difference in spring pad to wheel center line of the two axles.

If your wheels are RH5* then great idea to swap, if any other multi-piece wheel then likely you can use them if in good shape.

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-