When I converted my '55 1st GMC to disks I used an inexpensive kit on the front, used the rear axle from a car that matched the components the kit used on the front, and used a power brake dual master without the power booster, but from the same car, to complete a system with matched components.
Basically I had a '73 Chevelle wagon, used teh rear and the drum brakes under the rear, then bought the disk brake kit, and dual master mount from ChevyDuty, and bought rotors, calipers and master for the Chevelle. I also replumbed the entire system using the combination valve from the wagon and a pair of residual valves I ordered online from somewhere.
My brakes work great.
Where you are going to run into trouble is that there are no kits to put disk brakes on a 3200 as far as I know. Guys have found ways to adapt disks to the bigger spindles, but unless you really plan to use this truck as a 3/4-ton pickup, I'd consider finding a 1/2-ton front axle and swapping that in place and upgrading it to disks.
