I believe that vintage power steering still used the old slave cylinder on the steering linkage??? They never were very good, sometimes the adjuster on the box can be tightened, check a Chilton's manual for the adjustment procedure to make sure you do it correctly... If the box is tight and the caster and the rest of the front end components and alignment specs are in order about the only thing left to do is adapt a later model power steering box onto the car, or maybe one of the rack and pinion conversion kits for it. I do know the old Mustangs with power steering were a bit squirrely, another option would be too do away with the power steering and run manual steering, they still turn fine and probably the car won't do enough parallel parking to really need the power steering anyhow.... Let us know what you discover!!!!