I think inspection's tolerance on bumper lengths is pretty high for this very reason. I know mine was at Lone Star.
There may be another option if these bolt heads are your only protrusion outside the bumper zone. You may be able to use a rivet nut in the outside bracket and thread your bolt into this rivet nut. This may be quicker and easier than increasing the frame perimeter with plywood or other material. It would also leave your current robust bumper-frame connection intact. See this McMaster-Carr link for an example of some rivet nuts:
McMaster-Carr Catalog Page 3269
Only downside is you would have to modify the length of your axle spacers to accommodate the length of the backside of the nut. And you'd probably have to shorten your axle bolt slightly.