I have several P/S pumps saved off various engine swaps..
I'm pretty sure a few I saved that came on 1975-1979 straight sixes with the integral head had longer shafts for the pulley,but a few guys have told me the pump is the same as a V8,its the pulley itself that has a different offset --I haven't looked to verify that,they might be right..
Wont hurt to measure the shaft just in case there are some variations from one engine to another..
The info about the flare vs O-ring fittings is correct--you can swap the bung on the back of the pump so you can retain the original P/S hose...the hydroboost pumps have more pressure and flow and a different "can" with hose barbs for the extra return lines--I'm not sure if those pumps will bolt up in place of a regular pump,but probably will..
I have a few early P/S pumps that have a round "can" instead of the typical "canned ham" style,I think they came off pre 1970 vehicles..