You can download workable and searchable copies of both the factory service manual and the overhaul manual here.
http://67-72chevytrucks.com/vboard/s...d.php?t=558016
Really not sure you need rear disc brakes. The kits I have seen either don't use a park brake at all or rely on a poor mechanical design intended for a much lighter vehicle.
If money is tight get your 72 front disc brakes up to as new with some quality pads, rotors, calipers and hoses. They do most of the work even more so on a lightly loaded truck. Then just go through and clean up the rears. Adjusted and working correctly they do a great job.