[abisel]

May I offer a suggestion.

Remove the steel mounting arms from the pair of rear bumpers. Use a spot weld cutter or such to cut through the outer bumper skin. You may have to also remove the mountings for the bumper guards. Then clean up the mounting arms and attach them to the fiberglass bumper with rivets and layers of fiberglass and epoxy resin. By attaching the mounting arms to the car and then positioning the fiberglass bumper to the arms, you can get everything centered pretty well.