The fiberglass outer bumper will bolt to the inner-reinforcement, and if your chrome bumper is rusted out the reinforcement probably is too. If not you're lucky, if so it's a good opportunity to grab some aluminum reinforcements.
It's true fiberglass bumpers aren't the greatest idea on the street, but as long as you retain the steel (or aluminum) reinforcement it's not as bad in my mind.
It's the folks with fiberglass roll-pans and no reinforcement behind them that are putting themselves in danger...