[scottk]

67-68 birds with manual brakes used the same masters as the ones with power. 69 disc was power only.

power brakes or disc brakes in themselves will not decrease stopping distance, but disc brakes decrease brake fade.
Additionally, proportioning valves or other balance valving will not decrease stopping distance either, except by maintaining stability. Balance valving keeps the car balanced by decreasing fluid flow or pressure to keep wheels from locking early, and maintaining stability.

power brakes decrease pedal effort.

if you have single piston, 2 15/16" calipers and 11" rotors, i would recommend an 11" power booster, and a 1.125" master, because that is what the factory used on these cars.

the balance valving is most useful in the snow. under normal/dry conditions, it is not very useful on this specific application.
if you tend to ride the brake or do light stops, a metering valve up front will help get more use out of the rears.