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

I have a truck that I have run with 295 tires on 20 inch rims and 295 tires on 15 inch rims on all four corners, on the 20 inch low profile tires the truck jerks all over the place, put the 15"s back on and it stops doing it... And nothing is even close to out of spec on the front end. You would not risk taking your hand off the steering wheel on the 20"s.....that's my experience yours may differ.