Most Automotive PS pumps are a fairly useful little hydraulic pump... What are you planning to use/do with the system you're thinking about? More info would be helpful.

I do believe open center would be the way to go, that would keep you from spending all your time against the pressure relief valve in the pump, of course depending on how you plumb into the system, most of your fluid might well be going through your existing steering, which wouldn't do you any good. I guess I'm going to fall back onto my earlier... "More info needed"

MV