I doubt the 18" wheels you have are the Firestone RH5 "widow makers", I've only seen 18" GM wheels as multi piece wheels.

Nothing wrong with them if in decent shape, and you can find tires without trouble.

If you want a one piece tubeless alternative some 19.5" wheels probably fit, check that they fit over the drums.

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-