[Green Machine]

One thing is that those repop bumpers and licence brackets tend to warp the bumper up in the middle, your licence bracket is pulling the ends of the bumper down, making it high in the middle. I have had better luck on the repop tri-plus bumpers using a GM licence plate bracket, still available, they used the same rear one thru 1987. That is where I would start. Maybe the rear of the box needs be moved a little to the passenger side ? Make sure the bumper brackets don't have high spots where they touch the bumper, or they will put dents in the bumper when you tighten the bolts.