Rear brakes lock up, front disc not stopping well

You can switch to 7/8" diameter rear wheel cylinders to reduce their effectiveness.

But on my 68 cuda and 64 Val, I ditched ALL factory proporting stuff.
Ran the front brake line into a simple T. (Or plug the rear line port on the stock valve) Then one line out to the LF and another to the RF.
Installed a Wilwood (or Summit) adjustable proportioning valve in the rear line. A good place to put it is on the frame rails near the firewall.
You lose the brake warning light feature but who cares? If you develop a leak you'll know it without having a light telling you your foot is too close to the floor.