I see you now have a older Cardinal in your profile.
As glazier says the back of the pin box can be tapered. However the back of the pin box on many older 5th wheel trailers was squared. The fix was to cut more of a angle off the back of the pin box....and mounting flange if needed. I had a '97 5er that I had to cut a big angle for enough clearance between the tailgate and sides of the bed on sharp turns.
Give B&W a call with your problem. I've used their GN hitch when they first got started in the hitch business. They were a great bunch for helping their customers regardless of the size of the problem.