I presume the nut is tight which secures the pitman arm? Even so, there should be splines on the output shaft from the steering box. Check the following: 1. The splines and/or arm is ok at the output shaft. 2. The other end of the arm connects either to the control valve (PS) or another link (MS). A common problem is the control valve has a worn ball stud. 3. Is it possible the play is in the steering box itself? If so, you may be able to adjust out some of this, but usually the worm gear binds before the play is out. Kits are available from the usual places (i.e. Mustangs Unlimited, AutoKrafters, Mustangs Plus, John's Classic Cougars, etc..). There doesn't seem to be alot of places around which have a pitman arm for the 1969-70 model years, however. I've just been through all this for my car. You'll more than likely need a puller to extract the pitman arm.
Steve C.