There is a lot of info on this topic if you do a search on this site. Go to steering and suspension forum and search for topics posted by JIML82. He has some very good authoritative info for you. You will not be disappointed.
I don't know what your budget is, but I went with a new AGR unit though Summit, and for about $300 it's a great unit. It has a precise (i.e. "modern") feel, quick raitio, and what I like the most is that the valving gives the feeling of power steering at low speeds, but it almost feels like it's manual at freeway speeds. Nothing will compare to rack and pinion for accuracy, but the AGR unit comes close enough to make driving it fun again. I was already running lots of pro-Touring suspension/steering mods on my '70, and the AGR unit finally was the missing link to make it all come together.
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