What are you using up front to match these big rear brakes? Maybe 13" with Viper Calipers or something massive like that?
All the Mopars I have ever owned tend to lock up the rears way before the fronts break loose. I've had to use smaller rear wheel cylinders on the rears to "calm" them down before switching to small rear discs like Dr Diff's Econo rear brake kit in combination with the Mopar Police Taxi front brakes with Firm Feel pads. That combination is nearly perfectly balanced on both My A-Body and E-Body. No additional proportioning valve needed.
