Mark you made a good point and provided an excellent example.
Please explain the steps you took to select the correct dual reservoir for your cars/trucks.
My two toys, a 56 VW beetle and a 59 Fleetside long wheel base. On the VW it is fairly common practice to use the system from a later Beetle. Its been several decades but as I recall I used a 67 beetle master cylinder. I also changed the drums to larger size as the early Beetle brakes were not great.
The 59 is a mixed bag of parts with larger drums in the rear from a 69 GMC and front discs from a late 70's full size Oldsmobile. The Master cylinder and proportioning valve match a late 70's full size GM car, but using an 8" dual diaphragm booster because of space limitations. .
In both cases I just used combinations of parts that were similar or identical to those in later vehicles that originally had the dual system. The bug does not stop as well as i like and may eventually receive a front disc system, but the truck stops and tracks very well. I don't think it is rocket science, and the proportioning valve needs to either be adjustable or one that is from a similar weight vehicle with a similar brake system. Proportioning valve may not be needed for a drum/drum system.
This worked for me and both vehicles stop better than when they were born and with less pedal effort. Your mileage may vary, I am not advocating as an expert, just sharing my experience.