I've just ordered the 6915HKR's if you bring those up on the Summit site, there is a guy on there who left a comment saying that his fit in a 71 Mustang with power steering, this is what he had to say on the matter,
These headers looked great for a set of painted headers. I sand blasted mine and ceramic header painted them to fit the paint scheme i have on my 1971 mustang. It fit the power steering BUT you have to turn the steering just right to get them in place. Overall fit great once i have the pit man arm in the right position. Would recommend them greatly oh yea and they sound great.