You should be able to run disc brakes on the Rally wheels and also 71-87 6-lug wheels. The only problem I've heard of are some Wheel Vintiques rims and possibly with some drop spindles. Otherwise you should be fine. Not sure if you tried it, but do a little searching to find out for sure. Check out the threads below.
http://67-72chevytrucks.com/vboard/s...d.php?t=503975
http://67-72chevytrucks.com/vboard/s...d.php?t=398068