We just did a power steering "tech session" at our last club meeting. I have also converted my '64 from 47 turns L/L manual to High Ratio power steering. Here's what I've learned:
Existing Power steering to Fast Ratio Power:
The physical dimensions of the boxes and the mounting bolt patterns are identical.
Somewhere around '70 GM dropped the 4th mounting ear.
Late model boxes use the small diameter input shaft.
Late model boxes have O-ring type sealing.
The round "end cap" on the box determines the travel (turning radius) of the steering box. There are lugs cast into the interior surface that act as limit stops.
You must use the pitman arm from your car (year/model) to maintain linkage geometry.
Lee Mfg. makes O-ring to Flare adapter inserts to use a late model box with "old" style lines.
You can swap the end cap from your original box to a fast ratio box and maintain original travel (turning radius).
The late model rag joint will have to be slightly modified to work with the older column input (enlarging 1 mounting hole).
Your existing box can have High Ratio guts installed if you want to maintain "originality".
Manual to Power Conversion:
(this is my own experiences)
I thought the easiest route was this. I found a complete late model "donor" car (wreck - '79 Malibu w/ 305) and took the entire front of the engine off. All pulleys, WP, brackets, PS pump, bolts, etc. went in the wheelbarrow.
I scored a '98 Trans Am box from Ebay for $50.
Ordered the O-ring adapters from Lee ($12)
Bought a new rag joint (GM - About $60)
Bought a new PS Pitman arm from OPG (OUCH)
This worked out in my situation because I wanted to use a late model alternator, long WP, and clutch fan as well, and had already extended the alt. wiring harness to the passenger side.