Lots and lots has been posted on this subject.
If you just want to buy a rebuilt PS box and as I see you have a similar car to me, you will at least need a Saginaw 800 with variable ratio. TAG SPA-T or V. You could opt for a fast ratio upgrade as an alternative.
From all accounts, RedHead is one of the best. Lee Power Steering is another.
Personally, I rebuild my own, but not something to do if you're not very mechanically minded and have the tools and equipment to do it. A simple seal change is one thing, but a total rebuild, not so much.
As for newer steering options, adding rack and pinion can be done, but it's a MAJOR rework of the frame cross member. A friend of mine is swapping in a Coyote motor and to do it, he grafted in the complete front steering and suspension from a newer Mustang.
A good PS box with all the steering links etc redone, will give you as good as it can be. I can drive mine at 70 mph no hands.......... not that I want to do that for more than a few seconds!!
Hope that helps.