See if you can find out what the piston size is in the rear calipers.

The rear brakes should apply about 1/2 of the force of the front brakes. So in a 4 wheel disc brake application, that means that the rear calipers need to half 1/2 the piston area of the front calipers.

This rule of thumb should work fine for most street driven Mopar muscle cars.

The piston size in the rear calipers is very important. If they are too big, or too small, the car will never stop correctly. You can't fix it with a prop valve either. The front to back bias has to be correct before you tune anything else.