Need help mounting bumper on 66 cuda

Thanks retractable, I understand the idea of shims between the bracket and frame, that is not problem.
Let me clarify, the problem I am having is more of a bumper height problem.
With the bumper in its lowest possible position, the top of the bumper is tight against the rear body panel. I need to lower the bumper about 1/4 to 1/2" for clearance along the top. The only way I see a fix is to elongate the holes in the frame, unless I am missing some part of the bracket.