Ok, I hate to say bad things about a company that makes aftermarket stuff for our rigs, but...
I (and a frew others) had the same issue where the stiffeners didn't line up with the mounting holes, and after talking to them on the phone and the other people that had used them, it was decided to only use two of the mounting bolts (that secure the stiffener to the frame) and do them loosly until I got the bumper (in your case the hitch) back on and then torque all the bolts down...