Mounting fiberglass bumpers

I would bolt the brackets in place, use those rubber tipped long reach clamps, clamp them up and do lots of measurements and adjustments till you get them where you want them. Mark the holes thru the brackets onto the inside of the bumper. Drill a small hole to start and make sure to use masking tape to help keep the fiberglass from splintering. If you have a tappered reamer or even a dremel tool with a burr I would use that after the initial hole is made you stand alot less chance of busting or splintering. Good Luck.