Airflow will take the path of least resistance. As such, you want to channel all available air coming through your grille area to pass through the radiator to maximize cooling efficiency. This means any holes in the core support for A/C lines, wiring, etc. need to have grommets around them or to be blocked off if no longer used. Make sure the radiator block off plates or deflectors on each side of the radiator are in place. Fox Mustang deflectors were made of rubber and are usually missing from age or accident damage.