Check the eccentric bolt on the lower control arm. I had one loosen on a 72, and it did exactly what yours seems to be doing. The toe and camber of the front suspension changed drastically when it moved, causing sudden pull. If so, you may want to look into the "lower control arm camber adjustment kit" on page 2. Good luck.
http://www.sacramento-mustang.com/sm/catalog/susp.pdf#search='mustang%20%20eccentric%20eliminator%20plus'