The NV4500 might not have been in GM until 1993.
Have a look at it and see what it is, likely has a tag.

http://www.novak-adapt.com/knowledge/transmissions/manual/nv4500

•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-