I've seen plugs on other makes of trucks, IH and Ford I'm pretty sure. So in your search also check reproduction parts for those as well.

Leaving the filler neck in place and blocking the hole inside sounds like a very good plan. I think that looks better than a blank rubber plug.

•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-