I was looking at the linkage on the vacuum booster from the 72 dart. There is an arm that multiplies the movement of the brake pedal rod. For a given stroke of the brake pedal rod, that linkage gives more stroke on the rod coming out of the booster.
That explains why they used a larger bore on the manual cars instead of a smaller bore which is what you would expect. Because they needed more bore to move the same amount of fluid with a shorter stroke.
So does the 15/16" bore master cylinder move enough fluid without the added stroke of the power brake linkage? Maybe you just have be more vigilant to keep them fully bled?