Steering All Over the Place at 55mph+

67Mungo said
it would seem that the upper control arm and upper/lower ball joints are from an E Body, the brake system from an M Body, and no idea about the spindle

Sounds like you need to determine the parts you have.

If you have E body parts your suspension geometry is off and can induce all kinds of bad behavior.

I can drive my 67 dart at 70 and it is responsive and stable.