Rear brakes lock up... front disc

Jim, I'm having the same issues with my '73 Dart.
Front discs (11") and rear axle upgraded from the wimpy 7-1/4 to a 8-3/4" with 10" drums.
I don't know if the 8-3/4 has larger brake cylinders or not, but the rears lock up 'fairly easy' with my car aswell. Stock prop.valve.

I describe it pretty much the same as you do, it feels like the front brakes just aren't grabbing hard enough.

I've got an adj. propvalve, ready to try out, but since the front brakes need replacing (calipers/brakeshoes/rotors), I'm currently looking into upgrading to a larger discs and maybe Wilwood calipers to save some weight from the front.