Dugster,
I switched my 73 dart from power to manual brakes by simply, by replacinf the power booster with a manual brake pushrod from a 1969 barracuda.
I used the same master cylinder without even taking the lines loose. I just removed the power booster by unbolting the M/C , unbolting the power booster from firewall, and disconnecting it from the brake pedal. I then bolted the M/C to the firewall (where power Booster used ot mount and installed the manual brake rod from pedal to M/C. they work fine, the pedal effort is fine and the pedal stays firm when I stage the car at track!!
Go by a manual brake rod at you local used parts recycling facility (junkyard) and you are set.
Bob