If you do not have pics of your car with the original pattern to document your exact car, then what ever you do is a guess. The reason it was done was so no body color showed through the grill and under the bumper. Therefore you either shoot it sparce like the factory or complete like an over restoration.
Another survivor. 26,000 mile untouched car.Hardly any paint behind the bumper. Painted behind grill and very sparce below the bumper.
Oh and by the way the color used on the Challenger does not have enough sheen compared to the survivor pic I posted of the documented 5600 mile bee.