Brakes - as said, adapt later self-energizing drum brakes to the front or discs. Don't forget a dual master! Since these have underfloor masters, you have to decide if you want to adapt there or swap in a swinging pedal and mount on the firewall.
Engine/trans - a small block and auto will work, but on the cars, you have to move the engine over about an inch to the right and use A body exhaust manifolds to clear the steering. The truck will probably be the same. There are universal mounts or you can fab them up. Don't forget shifter, radiator, trans cooler and upgrade the fuel system.
Driveshaft and rear - Go with a later model rear axle as said. If you want a nicer ride, springs and shocks will need to be swapped too (front and rear).

When doing this, the next thing to concentrate on is electrical. 6 volt positive ground 70 year-old wiring is NOT good.

One thing that can save you time and money is to get a donor. Look for a good running drivetrain in a cheap (ugly or wrecked) body. I'd suggest a Dakota, you could use the engine, trans, driveshaft (fitted to new application) rear axle (measure width first!), wiring, seat, nuts and bolts, Throttle cable and pedal, maybe the gas tank, steering column if needed, and sell off the remains for scrap or parts.
I doubt the discs will work, but the master, booster, and pedal could.
The locations of the radiator, front axle, engine and firewall are NOT the same for the Dakota and an early truck, and the Dakota has a stepped frame compared to an early, but some have swapped the frames. It's a LOT of work, so don't think it's a bolt-in. Read some of the other posts in this section...