Tire wear, what's causing this! ? ?

The pitman arm has a master spline, and the steering box has IIRC 4 matching splines ; each at 1/4 turn. That is to say the splines are 90* apart. So since your steering wheel is so little out, you probably just need your tierods equalized. But yes its possible the P-arm was miss installed, but I highly doubt it.
-Plan1
Take it to the alignment shop, as is. They will center the steering for you. Its part of every alignment.As is adjusting the steering box. And if you ask for a 4-wheel alignment check. They will check the thrust angle. A bad T-angle will set your car, dog-tracking. This is not east or cheap to alter on a leaf-spring car. So I would specify to have them check it first, ask them to explain your options to correct it, and how much it will cost. Then make your decision accordingly. The thrust angle does not absolutely have to be corrected. Esp. for a DD .For the most part,only the guy behind you will know when its out.I only mention it because our As are somewhat notorious for dog-tracking.
-Plan 2
Get the car off the ground, high enough to slide under it.The vehicle weight will need to be on the wheels.This will require ramps or blocks. Ok,center the steering wheel and lock it there.Slide under the car with a tape measure. Carefully check the tierod lengths.The important measurement is between the ball centers.Since the ball-centers are encapsulated, and you cant even see them,you can use the studs. You may have to do some estimating. The important thing is to get them both at least close to the same. If your toe is pretty close now, then add the two measurements together, then divide by two. reset both tierod lengths to this new measurement. If your frame is straight and no steering parts are bent, and caster/camber is close, this will get your wheel close to straight ahead. Lastly check and adjust the toe, by adjusting each tierod length equally.That is to say what ever adjustment you do to one side, you must also do on the other side.
-Plan 3
-Heres what you do; A) remove the arm, B) center your steering wheel between locks, C) anchor it there, D)go turn your front wheels manually so that they are pointing more or less straight ahead, E) slip the pitman arm onto the nearest masterspline, F)With the steering wheel still locked straight ahead,and the wheels now pointing close to straight ahead, fine tune the position of the front wheels with tierod equalization, until the wheel look pretty good. G) set the toe as best as you can, and H) truck er off to the alignment shop.
I think its worth mentioning again; I think the problem is really just equalization.
I also think its prudent to advise you not to drive it this way. Since your steering box may be out of the reduced sensitivity zone, for straight ahead driving, theres a really good chance your car will feel twichy when driving straight, even with a perfect toe adjustment.It wouldnt hurt the tires or anything else. Its just awful annoying.
-Hope this helps.