You asked about a clamping plate, as referenced and shown here, page 27
https://www.accuridecorp.com/files/2012/10/Accuride-Wheels-Product-Catalog-Summer-2011.pdf
They're important as most of the GM wheels in question here required their use, your truck would have come with them originally, as did mine back in the 50's

2000 and prior GM used 8 lug on 6.5" bolt circle with 4.56" bore.
After 2000 I believe they switched to a center bore of 4.60 for most applications, but not sure if the step vans and related chassis kept the old bore or not?

Perhaps you have the much later model wheels, wrong center bore.

•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-