[rpmerf]

I'll add what I can here.

What is the use of this truck? Occasional driver, daily driver, hauling, towing, drag racing, track racing, autocross?

I believe you CAN use a drum / drum master with a disc brake setup, but you have to keep an eye on the fluid as the the front reservoir is a bit small for disc brakes. A disc / drum master will have a larger disc reservoir that will keep up with wear better. You will want to see what size master you should have on there. Stock master for power brakes should be 1-1/8". JB7 brakes form the 80s have a 1-1/4" master.

You will always need a prop valve to balance the braking between the front and rear. It should be designed for a disc / drum pickup. You could get a custom one and go someplace where you can lock the tires to adjust it.