Mike,
Welcome to the forum. Your first post got me off my butt so I scanned some of the sketches I made for the rear valence about 15 years ago. The patterns are for both back up lights and the exhaust cutouts. I used a 65 GT valence to add exhaust holes in a non-GT 66 valence, and that same 66 regular production valence, to add back up light holes in the 65 GT valence.
Jim