Preinstall check, which makes life much easier:
I was told by the tech line for hidden hitch that the brackets could be bent during shipping and could be bent back, but only as long as they are off by an inch or so (I was concerned about weakening the metal).
Perform the following procedure at your own risk since you may be doing some bending and will be making judgment calls on your own. Call the hidden hitch tech line to verify all the info below
Measure the mount holes center to center from one side to another. They should be 52 inches on center. Now measure the distance from one of the center mount holes (above the receiver) to the last mount hole on each side, again on center. Should be equal distances on both sides.
If not figure out where they are off. The mount angle brackets should be square with the hitch. If either look bent in or out you can bend them back with a 4x4 or 6x6 block of wood and a sledge hammer. Rest the bracket on the wood block and use the hammer to persuade them back to correct dimensions. Bend them back until they look square and straight and the mount holes are 52 inches on center from one side to another and are evenly distanced from the center mount hole above the receiver, on center. The techline at hidden hitch did not recommend bending them into place if they were off by more than an inch or two. Again, you can check to see if they are bent evenly by making sure the distance from one of the center mount holes (above the receiver) are of equal distance to each side (last mount hole).
I did this procedure because I noticed one bracket was bent inwards about a half-inch or so. After following this procedure, the holes lined up perfectly and installation was painless the first attempt.