A-Body 1968 manual steering pitman arm spline alignment
There should be only one way it goes on.
But...
Turn the gear box lock to lock
Count the number of turns.
Position the gear box on the bench (or install in car) so it is in the as installed on the car position
Rotate the gear to one lock and return 1/2 the total number of turns
You should be able to install the piston arm in the correct location and the splines should line up correctly.