Rear brakes lock up... front disc

My 67 Notchback Barracuda has stock Kelsey-Hayes power front disc brakes and rear 10" drums on a 7-1/4" rearend. It stops fine most of the time except in hard braking the rears will lock up and send the car all over the road. It's really dangerous so I have to fix it. I've already tried smaller slave cylinders at the rear; they helped but have not solved the problem entirely. The pedal is firm and at the top. Any suggestions?