Brake upgrade questions

My 75 Dart Sport was originally a power brake car. At some point it was converted to a manual brake setup.

I am about to commence with the rear axle rebuild/brake upgrade. Beside replacing the open diff with a Sure Grip and swapping 2.45s for 3.55s, I am adding the Jeep Rear disc brakes.

Since I am opening the brake system, I figured it's a good time to replace the master cylinder. So I have two questions:

Are power brakes even a benefit on these cars? I have only driven "A"s with manual brakes, and they seem to do fine.....

I am looking to put on an aftermarket master cylinder with the larger 1 1/16 bore..will this effect my braking in a negative way?