You can get an adjustable prop valve from summit/jegs/napa/etc and set it up to have the correct front back bias (but quickly allows you to change it if you should swap to 4 wheel disc).
That said I would first try changing the M/C, just go to the local autoparts store and ask for the off a 73 340 duster with power disc brakes. That will be right one. Which newer aluminum one, I couldn't tell you off the top of my head.
Oh one other thing, was your car an original power brake car? You might have a manual m/c on it and not a power brake m/c