Jbutch. Be very careful what you use to fill the gaps. Bug screen tend to have very small openings and will restrict air flow over the radiator and AC condenser. Most of us used 1/4" wire mesh cut to fit the entire opening from the inside of the grille. Works great to keep bugs and debris from hitting vital parts. Check out the link that Kswhodat shows and you'll get a very good idea.
I did the 1/4" galvanized screen on mine. Cost was under $20. Get a roll from local hardware store and spend an hour and you're done! I was tempted to spray paint it black before installing, but it looks good with the silver mesh behind the black motor doors.