Watch out that you don't run out of pedal travel at the pushrod if you go to a smaller master cylinder than the one you have. The underfloor pedals typically have a pedal ratio of 6 or 7 :1, and the distance they can move the pushrod is limited by the shape and size of the pedal. Disc brakes do require more volume of fluid to be delivered to the caliper, hence the larger bore and power assist requirements.
If you can get 1" of pushrod travel with your current setup before you lock the brakes, , when you go to a 7/8 master you will never lock them. If you were running a different caliper, such as a Baer or Wilwood with smaller pistons, the smaller bore would work.
I try to set the systems I build so they lock the brakes at 3/4" of pushrod travel, and the full pedal travel will move the pushrod at least an inch with the bleeders open.
I've done several swaps using the same setup you have, and the 1" and even 1 1/8" seems to work alright with the underfloor pedals, and 7" booster and even as low as 16" of vacuum, but there is some extra pedal effort. (how much extra? hard to define, but to me it was acceptable)
Don't forget, when you decelerate, your motor will develop much more vacuum that what it shows at an idle. And when you hit the brakes, (unless you are doing brake stands), you should be decelerating.
I just put an auxilary vacuum pump on a guys car with much the same setup as you, he had about 15" of vacuum, it made the brakes marginally better as far as pedal effort, but the noise it makes is undesirable...( it sounds like a cow making a moo sound.)
I have looked at those vacuum/pressure charts, and found them to be somewhat exagerrated.
If you have a disc/drum setup, you do want to use a metering valve in the front brake line, these are found in a disc/drum combination valve or you can buy them as inline stand alone units , which double as a "Y" for the front brake line.
The adjustable rear proportioning valve is just a pressure limiting device, and does not care if it is for disc or drum.
my signature lines...not really directed at anyone in particular..
BE different....ACT normal.
No one is completely useless..They can always be used as a bad example