Swap in rear discs 70 Dart

DoctorDiff sells everything you need. 15/16” master cylinder, adjustable push rod. You can use the factory combination valve, or replace it completely with an adjustable one, or just plumb an adjustable valve into the rear line. Having an adjustable one will be better balanced if you tune it well, but plenty of people use the factory valve too. Depends on how your car is set up and how you drive it.
I just went through this myself. Converted to BBP by purchasing Cass' axles. Was going to go rear disc but would have to buy his new brake lines & form/flare. Don't have the tools for that so went with the drum brakes (10x2.5) backing plate, shoes, and all hardware (bought on ebay). Existing master cylinder and brake lines did not need to be changed. Unlike the tapered bearings, I was a bit surprised that I had to drive the new axles in (could not push them in). Cass said to coat the outside of the bearing with some grease or diff fluid, then tap them in using a wood block and 2lb sledge. They popped right in. Since they are so tight fitting and have an o-ring on the bearings, should not have any more axle leaks.