Ok, here is some more info, sorry I've been away for a few days.
The manual MC part number for a mid-70's Malibu with disk/drum brakes is 1639. This MC has the 15/16" bore. I have not tried this one yet, so don't know how it works. I plan on putting this on my Nova, but I need the Chevelle to be running again...
For a 4-wheel disk brakes system WITH power assist, I have tried the MCs for both an 80 Corvette and 79 TA, both felt the same. The Corvette part number is 1749 and the TA part number is 1745. They operate the same, but the Corvette one looks like a typical Corvette MC that has two rounder looking bowls versus the TA one that looks just like normal disk/drum MC.
The other option is MANUAL 4 wheel disk brakes like I am putting on my Chevelle right now. The part number is from a 70's Corvette and is number 1371. Again, this one looks like a typical Corvette style that has the rounded bowls.
The issue of proportioning valves comes up a lot and here's what I think of it. For a disk/drum system, any prop valve from a like car should work fine. The important thing is to make sure the prop valve matches your system.
For a 4-wheel disk brake system, I see several options. First, if the car has good bias and doesn't transfer a lot of weight during braking, some people have said you can run without a prop valve at all. The big safety issue with any brake system is to make sure the rears don't lock up first, so running without a prop valve seems dangerous to me. The second option if you can live without the brake warning light is to run an adjustable prop valve in-line with the rear brake line. Then you simply need a line 'T' or splitter for the front brakes. If you like the idea of a brake light, you still have several options. First would be to get a prop valve for a 4-wheel disk brake car, either new or used, like the 79-81 TA, or Camaros that had 4-wheel disk brakes. That would probably be the best solution, but can be hard or expensive to get. The other option I am persuing is to use the 4-wheel drum brake distribution block and then put an adjustable prop valve in the rear brake line after the distribution block. I have heard this will work, but probably won't test it until Sat. The theory is that since this block only splits the front line (gives a nice place to hook both front lines to) and passes the rear line straight through, it should work fine, but gives you the brake warning light hook up. I just about have everything hooked up and in place, so I just need to bleed the brakes and test. I'll keep you guys posted as to how it works.
One last thing, IMO, the pressure required to operate the disk/disk manual system should be the same or less than a disk/drum system, but I won't know for sure as I never had manual disk/drum brakes on the car. One thing is that pedal travel will be more on a disk/disk system as it takes more fluid to push out the caliper piston versus the drum cylinder pistons.
Hope that helps,
I'll be back later...
Bill C.