The Super duty rear is a Dana 80, with either 4.88 or 5.13 ratio, Pretty much nothing off of it will easily swap to a standard truck, the spring spacing is the same as a F350 cab and chassis dually(narrow frame) though.
For discs for your truck, either aftermarket brackets with chevy calipers and 90's E350 rear rotors or adapting the entire E350 brake setup are the best options. Being you have the driveline brake, the $50 brackets and chevy caliper bolt on setup would be the way I'd go.

But, for what you're out to tow, a good working set of drums will be fine, and hydroboost will be plenty.
I'd grab the pedal out of the 91 if nothing else so you have it to compare to your existing one.

With that ZF, make VERY sure the seals in the brake assembly are good and that the oil is full. I know they have a very small oil capacity and they are very expensive to rebuild if they run low on oil.