1968 Dart Brake and Booster issues?

Here's what I would do, with 235/275 tire balance, or any balance of fronts being 85% of the rear;
Skip the adjustabe P-valve. Skip any P-valve. Fix the leaks.
With a brake booster you can run any sized M/C bore that you want to.
Iv you have a Disc/drum M/C with the one larger reservoir; Make sure the hold-back valve is removed from the port at that larger reservoir, and plumb that reservoir to the front brakes.