I believe it is supposed to be a tight fit-but probably shouldn’t rest up against the bumper (for reasons that you cited).I will try that, but I could feel the tips pulling tighter against the bumper as I tightened the clamp. I will grab a couple of paint stir sticks to use as a wedge and see if I can get a gap going. I would not mind them touching, but they occasionally "knock" against the bumper.
The tips on mine sit about 1/4” off the bumper, which provides enough clearance to keep them from hitting. I would think if necessary you could slightly bend the hangars or possibly add spacers/shims to bring them down 1/4” - 1/2”. Also make sure the pipe is slipped on all the way, and the clamp is torqued accordingly.
Once everything is tightened down, there shouldn’t be much movement.
Let us know how it turns out, and how you like the sound.