They should slip on the spindle. Press into the hub.
The ID measurements you provide for old and the potential replacements illustrate just why and by how much they don't fit. This shouldn't be a surprise given that info.

I haven't worked back from the cup & cone set numbers I provided to compare to the separate cup and cone numbers your parts guy found.... Are they the same?
And what about the advertised dimensions?

•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-