Tire wear, what's causing this! ? ?

I have a 68 S-clone. It has a 70 Duster KH set-up on it and 73 steering parts. It all fits.
The 68 parts are OEM; control arms and balljoints. These interchange up to and including 72.The Duster parts are;Knuckles,discs,and calipers, and 340K-frame. The 73 Dart parts are steering only parts; Centerlink,and all tierod parts, and the arms. The centerlink and arms must be treated as a set; they are different between groups of years. The later style(73 up) drops the centerlink for extra oilpan clearance.
As long as your new pitman arm physically fits on the sector shaft (there are 2 sizes), and has the pin oriented up/down (there are 2 styles) the same as what you are replacing,and the overall length between the pin and the splined hole are the same, other manufacturing differences shouldnt matter. The only important stuff is that it fits,and the length is about the same, and that the working angles are similar.
Hope this helps.