Unless you're considering putting a whole front clip under your cars front end to properly accomodate a R&P, I'd opt for keeping the original suspension with a similar style steering box that's currently on the car. The problem with retrofitting a R&P with the original suspension is the front sump oil pan on the Y which leaves only the area behind the engine crossmember to work with which in turn makes the turndown from the steering shaft to the rack too tight an angle to work smoothly.I am told that the '56 boxes are internally superior to the '55 models but you'll have to work around the shorter steering shaft if you went this way. I do know of someone local with a '55 Sunliner that's put a '56 box on his car with the improvement in boxes in mind but I haven't paid enough attention to what He did with the difference in lengths with steering shafts.
Jim Culver is currently putting a later model Cougar steering box on his '56 Club Sedan but is having to fabricate a 'new' pitman arm to work with the existing steering links.