Rear brakes lock up... front disc

Mother Mopar has long been noted for their one size fits all proportioning valves that almost always cause the A bodied cars to prematurely lock up the rear brakes under hard braking or less than optimal traction conditions (wet pavement for instance), resulting in the car's tendency to spin like a top.
Making sure the metal rear wheel cylinder brake seal expanders are in place or that you have residual check valve(s) in the rear brake lines, gutting the proportioning valve section of the combination valve, and placing an adjustable one in the line going to the rear brakes, and taking the time to do the testing and adjustment needed to get the settings right after you install it is the usual time tested fix.