Need help mounting bumper on 66 cuda

Heres the deal. I put patch panels on the lower rear quarters and before I do the finish body work I want to fit up the rear bumper to get the right spacing between the body and bumper etc. Well I put the bumper in place with the two outside bolts, and as I tightened the bolts it drew the bumper up and before I could get the bolts tight it hit against the rear body panel. The holes in the frame are slotted, and the bolts were at their lowest point in the holes.
The bumpers were off the car when I got it. Is there some part of the bracket missing? Should I just enlarge the holes with a die grinder?
Seems pretty straight forward but I can't figure it out. Any help would be greatly appreciated.

Bob