I suspect that a number of small items are adding to this pile---------

Incorrect bushings and/or improper mounting--check out a T bird ( maybe also Lincoln) to see EXACTLY how they are mounted-probably against a crossmember that can't twist or flex.

If the steering/suspension geometry has even one little varient incorrect it will take a significant amount of force to turn them and that will cause the back feed that will necesstate enough torque to twist the housing(applied thru the spiral/bevel to rack will cause rotation in that direction mentioned.

And -------are you setting down????? If the power steering is powered it will generate the force internally in the box and this won't happen-I don't think you have a problem