Rear brakes lock up, front factory disc 67 Barracuda

I've read that most factory Mopar's (be they drum/drum or disc/drum) are bias to the rear. The fix is an adjustable proportioning valve in the rear line to limit (adjust) the line pressure. This is considering you have a healthy brake system with no bad parts. See the attached picture of what you need.

I believe in your case there should be two factory valves, a second one to the rear. Did you install that as well?