There was a guy in NC that sold those kits. I think he was advertising in Super Chevy. He had a website and charged about $400 for a kit. Basically, he was taking 5 lug discs from trucks and redrilling them for 6 lug. and throwing in a bracket that let you mount a caliper to the drum spindle. Most of his parts were from late 70 trucks except for the bracket for the caliper.
I think mpbrakes and/or stainless steel brakes also sells these kits.
Here's one link.
http://www.performancesuspension.com/TruckBrakesGM.htm
Let me dig through my notes and see if I can find the specific contact info.