My 1956 Ford Ranch Wagon with 312 engine and power steering wanders at speed and the steering wheel will not return to center by itself after a turn.  I replaced both tie rod ends and checked the steering box for play with the pitman arm detached.  I've also replace the power steering valve and ram units.  The idler arm has no play in it.  I had an alignment shop say that the alignment looks okay but it acts like the caster may be incorrect.  Any ideas?
Thank you,
Dean

Usually, when one won't return to center is indicative  of not enough positive caster or excessively worn steering part(s).

Bias ply or radial tires? MS or PS?

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Did you replace any of the ball joints recently?

When I rebuilt the steering and suspension on my 55 Tbird, I was not going to replace the ball joints. Why? Gil Baumgartner (CTCI Gil's Garage) told me that the original ball joints on 55-57 Tbirds were over-engineered, bigger & stronger than they needed to be and don't wear out. Unfortunately, I had to replace the upper ball joint on the passenger side because I stripped the threads and could not remove it without cutting it off. The replacement ball joint was made in India. When I installed it, the passenger-side spindle required excessive force to rotate in and out. Although my car is still undergoing restoration, I could see where that would prevent the steering from automatically returning to center (manual steering). I researched the problem and found that it's not necessarily uncommon, and it's possible that once the car has been driven, the problem should cure itself. On the other hand, I have a NOS ball joint that will replace the aftermarket ball joint if necessary. 

I believe the caster is off and will have it realigned.  Running radial tires.  Thank you for responding.
Dean

There doesn't seem to be any wear in the ball joints.  No movement lateral or vertical movement when you test.  I replaced an upper some years ago and both sides seem to be okay.

Look for an older SPRING AND ALIGNMENT SHOP to hopefully have it done correctly. Today's alignment shops and techs have no idea of how to align older iron.

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When I first got my Tbird, it wandered easily. I initially put it down to cross ply/bias ply tyres. I later discovered that the steering wheel was set 1/3 of a turn off centre. The tie rods were adjusted incorrectly, with one extremely in and the other extremely out. Once I corrected that it fix the self centring issue.