Dave - If you stick with 13" wheels, try to tuck the tail pipe/s up as far as possible.
I have more than my fair share of speed humps around here and have leant to proceed with
caution.
Depending on the steepness of the hump I've managed to catch both the lower valance at the front and/or the tail pipes at the rear.
I like both your foam arch shape and the even gap around it.
Good luck, Paul.
