You should try the gears you have first before changing rear axles.
It’s sometimes difficult to rely on other peoples explanations on “best gear ratio” because so many variables, big ones being driving style, local terrain, and how you use the truck, what you haul, or always empty...

If you want to change axles anyhow then the GM 14 bolt is the most logical choice for many reasons.
Info here.
https://www.stovebolt.com/ubbthread.../14-bolt-axle-swap-info.html#Post1249600

•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-