for what it is worth...
i was trying to mount the rear bumper, but everytime i tightened everything down, i got a huge gap in the middle (due to a bow) and the sides kept chipping the bumper filler.
the way i resolved this (with the rear valance on)...
i bolted the bumper straight to the bumper mounts, and only let the rear bumper guards have about 1-2 threads of grip.
bolted the bumper guards to the rear frame (the bumper is now in the right position, with no gap and not chipping the rear filler)
then, bolted the bottom of the bumper guards as tight as i could (had someone wathcing the bumper to make sure it didn't move--it didn't)
last thing i tightened was the upper bolts for the rear bumper guards...
this process locked everything in the right position, and appears to be rock solid.
no bow in the center of the bumper,and no interference between the bumper and the rear bumpe filler...
hope this helps!
PHC