Brake Booster For 1971 Plymouth Duster

With front disks and rear drums, you most likely need an adjustable proportioning valve. You plumb it in series in the rear brake line, after the "imbalance warning valve" if you still have that. You adjust the knob until the fronts skid before the rears on a wet road or pea gravel. Cost $25 for Chinese to $80 for name brand. I use Chinese (ebay) and looks like from the same factory.

Above is independent of whether you use a power booster or not, as cudajim said. I understand that most factory systems used a slightly smaller MC piston with power brakes in A-bodies. I think that is because of the lever ratio added by the stand-off brackets. But there are many factors in selecting MC and wheel cylinder bores, including how strong your leg is. Also, a booster won't help much if you have a radical cam and not a steady and acceptable vacuum at idle.