Cuda front license plate bracket - what's correct?

I have a rubber bumper on my '70 Cuda. I am using u nuts on the bracket - but I have them flipped so that the protruding side of the nut penetrates the holes in the bumper (from the backside of the bumper) because it seems to keep the bracket/plate more stable.

But the plate angles back too far and is almost touching my valence filler panel and there is still play, so when I drive, it will definitely touch.

What is the correct way to secure the bracket? Shouldn't the plate/bracket be immovable once bolted on... or does the rubber bumper complicate matters?

Really? Nobody knows?