Don't take this wrong....BUT are the stock drum brakes that bad? I thought the truck manufacturers went to rear discs to save money, not for better performance. Big rigs still use drum brakes on the trailers so they aren't really obsolete.
What was the intended gain?
Disc brakes have better stopping power, don't fade like drum brakes, work when wet and are easier to service. You can't compare semi trucks to passenger cars or light trucks, totally different.
Since the trucks rear brakes need complete rebuilding along with new drums the cost and work involved to convert is nearly the same. So converting is a no brainer at this point.
I have been continually updating this truck to more modern systems including a third brake lamp, lock up torque converter, modern style wiper arms and blades, lighting, A/C system and more. The brakes are just a continuation of the effort. A hydraboost system in place of the vacuum booster will be next on my list.
