1970 Dart Manual Steering-Car jerks left and right when trying to drive straight...

I had some 245/45-15 F-somethingorother on the front of my FormulaS one summer.The car cornered real nice with those.But the hunting was miserable. I was working as an alignment tech at the time, So I tried about everything imaginable to get rid of that.Those tires were pretty soft and they only lasted a few weeks. I replaced them with a new set of 235/60-14s. Problem solved. Same alignment.