Sounds like you've got it figured out.
7.50-20 are still common enough.

7.00-20 are also still available http://www.millertire.com/products/.../7-00-20-firestone-truck-hwy-rib-10-ply/
look there for other sizes too, also try these people
https://www.universaltire.com/truck-and-military.html

•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-