While you are talking to the folks at Novak inquire about parts availability for the newer (starting in 1968 I think) SM465.
You may want to consider swapping to a SM465, it replaced the SM420, and is essentially the same transmission, but not actually the same... confusing right?

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-