[Christ]

Do a complete grille block, remove sources of external heat from the radiator. I.E. Secondary header shroud... Use a stronger fan, or a pusher fan, or add another fan to the open area of your radiator that isn't currently occupied by a fan (if you removed A/C fan) or wire the A/C fan so that it comes on with the coolant fan. This will help to draw more than enough air from places other than the grille to keep the car cool.

If you don't believe it, or need help realizing if it will work for you, do it, then drive out to a backroad on a hot day and let the car sit idling in the sun after driving it kinda hard. If the fans come on and never turn off, you need to channel more air into that area, which can be done with ducting rather than affecting the aero of the car.