With those modifications planned just keep the 4 speed and give it a try, you'll probably like it just fine.

If you want to spend more time and money for another small gain research and try an NV4500 five speed overdrive transmission.

The other 5 speed OD that is an easy fit is not really large enough for a 1 ton is the T5 transmission from an S-10 among other things.

•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-