Your upgrade to power steering outside the frame and using a later model Saginaw
steering gear should not wander like that UNLESS the shop did not provide the
appropriate amount of caster to the axle. If all the other geometry they did is correct,
then you need a good 5-6 degrees of caster.
Get the caster and toe-in done at a quality alignment shop.
Caster shims are cheap!
I had the same set up in my panel truck with 455 Buick and steered beautifully.
