You have a mostly original 6400 right?
If so then you don't actually need a working spedometer, as you will almost never be going over the speed limit.
I drove my 52 3800 around the country without a working spedometer, never once wished it was working. I did have a little digital tachometer made by http://www.tinytach.com/ it was handy.
As far as inspection, just don't tell them it does not work, They have never checked to see if mine works during an inspection.

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-