I have a adjustable proportioning valves in the rear brake lines on both of my cars along with the factory proportioning/distrubtion blocks.
I can tell you it is a very worthwhile addition to your brake system. My cars will stop on a dime and all four lockup togeather. I mounted the one in my 74 under the dash and I can adjust it on the roll, even while braking and can feel it working. I would never drive a old Mopar without one, it makes that big of a difference, I'm here to tell you. The 74 has had one since the middle eighties and it still works today.
You can see it under the left side of my dash in this picture.
I didn't want to run the steel lines through the firewall on my 71, so I mounted it here under the car. It works really well.
Kept all the factory valves in place for a 71 disc brake car. I believe you must have the disc brake valves in place to work well for disc brakes?