I agree that something is wrong and its not the MC size.

In my 67 Coronet I have 76 Volare 2.75" calipers with 10 x 2 1/2 rear drums (don't know the wheel cylinder size). These are manual brakes.

I run the Doc Diff 15/16" master cylinder and my pedal is just fine. Does not have excessive pedal travel.

67 Coronet 500 9.610 @ 139.20 mph
67 Coronet 500 (street car) 14.82 @ 94 mph
69 GTX (clone) - build in progress......