Welcome PrepperGreek!
The first thing I would do, buy a 1962 maintenance manual. Not the supplement manual.
https://classicparts.com/1962-shop-manual-gmc
On the inside of the glove box door is a Parts ID plate. This will list all the major components of the truck (axles, trans and so on).
The truck has Vacuum hydraulic brakes, that is what the extra port on the master cylinder is for.
As for the brake cylinders you will need to know what axles the truck has, front and rear from the ID plate.