If it is a 40 year old steering, then there is a good chance there is wear in all the parts as has already been pointed out.
Each piece has to be analyzed throughly for slop, movement, then proceed to the next joint.
When you get to the Control valve, the Ball stud has to move approx 3/8" total travel left to right. Don't get alarmed to see movement here, it has to move to move the spoole inside the valve to direct fluid movement to the Cylinder, either pushing, or pulling.
Of course excess movement does indicate a worn control Brass sleeve, Seats, Ball stud.
The Box, Remember, any movement in the imput shaft will produce a movement in the Pitman, Absolutely ZERO exceptions.
The calibation on the box is done at High Center (Exact midlle of the turning radius---4 turn box equates to 2 turns from hard lock left/right, 5 turn 2 1/2 turns gets ghigh center.
Dan @ Chockostang
chockostangclassicmustang