power or not

Just run manual. There’s no need for power brakes on these cars as long as you size the master cylinder bore correctly.

I've driven A and E bodies with both power brakes and manual brakes, and I've converted all of them over to manual disks with a 15/16" master cylinder. The pedal effort isn't that bad at all with a 15/16" or 7/8" master cylinder. The pedal stroke is slightly longer, but still no where near the floor. And the slightly longer pedal stroke allows for better brake modulation, not like the original 1-1/32" masters that make the brakes almost an on/off switch with a high and hard pedal.

The 15/16" master cylinder that DoctorDiff sells works great on these cars. My Duster is currently still a disk/drum car, but with an adjustable prop valve in the line to the rear you can dial in what you need to the rear disks.