I thought the SM465 had a 5.125" indexing diameter?
And if your speculation is correct that the early NV4500 has the same.. Then it's not a far stretch for someone to assume it bolts up. Some extra bolt holes may have been drilled and tapped in the clutch housing?
Not saying this is all true, just that some more investigation is needed before completely discounting the claim that it is an NV4500.

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-