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If you haven't bought the GT500 valence yet, contact Tasca ford (or similar) to make sure you receive the right one. I ordered it on my own, using their website, only to find out ford's catalog was written by 5 year olds... items weren't described clearly.
When I installed the rear valence, no modifications were needed. Removing the plastic clips from the stock valence were a pain. And for some reason, I couldn't install all the clips back on the GT500, but enough to hold the valence in securely. So, I ended up with extra clips as spares.