You can swap in disk brakes from a 69-70 full size Pontiac if you can find them, the problem there is the rotors are nearly impossible to find new. The most popular choice these days is using one of Scarebird's disk brake kits:
http://www.scarebird.com/12802/21632.html
Do a search in this forum, there have been numerous discussions on this kit and I believe there are some photos of the installation as well.