Front end geometry

Check your lower controll arm piviot where it goes into the k frame. The weld is known to crack. I had a 72 Demon that had that problem. I bumped a curb backing out of the driveway one day, it cracked and then it drove EXACTLY like you are describing. Took the torsion bar and lower a arm out, rewelded, problem solved!! Good luck and let us know how it works out!