You may find some useful info in the Tech Tips section of this site, link at very top of page, or here
https://www.stovebolt.com/techtips/

One of those is about the P-30 rear axle swap.
https://www.stovebolt.com/techtips/bigbolts/p30swap/

A more approachable front brake upgrade (than in the above tech tip) can be found here, sticky thread in the Big Bolts section.
https://www.stovebolt.com/ubbthread...4-1-1-5-and-2-ton-trucks.html#Post725572

You can find the gear ratio by counting turns of the driveshaft and wheels, don't need to disassemble anything.
First assume it has a standard differential, no locking device. If that is the case (should be for your model) then leave one wheel on the ground, jack up the other and because of the differential action the wheel that turns will turn twice as fast than if both turn.
Turn the one wheel 20 times and count turns of the driveshaft. Divide the result by 10 for your gear ratio, for example 61.7 turns of driveshaft is 6.17:1 ratio. Others may advise to only turn the wheel once, or turn both wheels together once.. the method I have described is both easier and more accurate.

•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-