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