1970 Plymouth Duster 340 came from the factory with standard equipment manual disc brakes.
On an A body, there really is no need for power brakes.
A good manual disc setup stops an A body very well.
Semi-metallic pads, properly functioning calipers and the rest of the setup in good working order, and you have an excellent braking car.
Pedal effort will be determined by the bore size of the master cylinder.
But power brakes with a vacuum booster are just added weight and complication on a 3,000 - 3,200 lb. A body. Just another thing to worry about when you run a decent cam that may not bring enough vacuum for the power brake booster.