Just talked to 'someone' who runs a high-performance brake company and has about 50yrs experience working with braking systems and components. He asked me not to use his name (again, due to liability), but was willing to take a few minutes out of his day to answer some of my questions about how to select hydro-max components for this application. Realize that this information might be a little more credible if I was able to cite the source, but was lucky to get any assistance at all. So anyway, here's what he told me:
The hydro-max master cylinder / booster setup in the medium-duty trucks is pretty much the same (unchanged) over the years, so the only thing we need to be concerned about is selecting a unit with an appropriately-sized master cylinder. And regarding the size of the master cylinder, he didn't care about the diameter of the wheel cylinders or the brake drums. Said that since we're sourcing parts from other applications, and there's not much to choose from, there's no point in getting into the details (he's a busy guy). The long and the short of it is that there are 1 3/4" and 2" bore master cylinders available, and he said that we'd definitely want to go with the 2" version. But he couldn't tell me off the top of his head what applications had that 2" master cylinder.
So that's what I'm on the hunt for now. A 2" master cylinder/booster unit. Would prefer to buy new/rebuilt, as I'm planning on adapting these parts to the stock pedal assembly, so don't need anything else, and would rather not use junkyard parts. If anybody can help me source parts, I'd sure appreciate it. Know a lot of different trucks used these brakes, and imagine that prices for replacement parts are going to be all over the place. Ready to buy parts over here...
Oh, and one last thing. He also confirmed that Jake0147 was 100% correct about pressure being equal at all three axles in a two-circuit system (front axle / rear axles), as long as the master cylinder doesn't have a step-bore. Jake's explanations were very clear and made perfect sense, but it never hurts to get a 2nd opinion.
Full steam ahead. Let's get this figured out!