I was just told yesterday that the grilles were not interchangeable. Something about the grille openings being 1 inch smaller. I think the bumper area is wider so the grille could be 1" smaller on the bottom side of the top and the top side of the bottom.
I'm very interested to find out if that's accurate or not.
I compared the grille on a '13 Roush Stage 3 with a '12 GT500 and the bumper section on the Roush is DEFINATELY wider. The lower was without a doubt smaller (on the '13) but the upper looked close. I'm just not sure if the Mustang GT (and/or Roush Stage 3) is the same as the GT500 (facia).
Please let us know what you find,
The shop told me that my car has already the grilles installed. I will go today and pick it up. Post pictures tomorrow