Depending on your ability to fabricate caliper mounts, the simplest solution would be to find a donor truck with 8 lug hubs and scavenge the entire brake system, rotors, calipers, master cylinder, and maybe even brake lines and proportioning valve, etc. and adapt them to the truck you're working with. Trying to mix and match components from different sources can get frustrating, expensive, and deadly DANGEROUS. Let some manufacturer's engineering department do the R&D and steal their ideas! Fortunately, virtually all 3/4 and 1 ton trucks use the same 8 lug bolt pattern for the wheels, drums, and rotors, so don't limit your parts search to GM vehicles. It's entirely possible a system from a Ford or MOPAR might be a better fit. Once the disc is mounted to the hub with a few lug nuts, use air pressure to hold the caliper in place and fabricate a mounting bracket from appropriate-thickness steel plate to connect the calipers to the rear axle tubes. If you have some skill with a plasma cutter and a welder, that shouldn't be a particularly difficult task. Be sure to account for the side to side movement a caliper needs to make in order to compensate for pad wear when choosimg a wheel with the right clearance for the caliper.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
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