I remember there was a big discussion about this when Dave Walden was restoring his Challenger. I don't remember all the specifics and I'm not sure if that info was saved in the archives or not. I seem to remember him saying that the rear valance was installed "before" the car was painted body color. The front valance was not installed but was hanging in front of the car or in the windshield opening while the body was painted. Both should have some body color overspray on the back side.(probably more on the front then the rear) I also seem to remember him saying that the front valance was installed "after" the radiator blackout since there was black paint in areas "behind" the front valance. Not sure about undercoating though.

I know Challengers had the back side of the front valance blacked out.(in the center) Not sure about if Cudas were the same way.