Are you looking for true fog light function, or just looks? Placing the fogs in the grille give the retro 60's/70's GT look, but do not operate well as fog lights. Placing the fog lights as low to the street as logically possible will give you the best fog light function. Yellow is the best color of light to use as well. The low positioned fog lights produce less light splatter in the fog mist. Ford offered a lower valance with fog option for the GT's, and block off plates for the upper grille (or these openings where used to provide vent tubes to the front brakes on the Boss Mustangs).
If it's for looks, then its all in the eye of the owner.
It is better to keep your mouth closed and let people think you are a fool than to open it and remove all doubt.