I can't say for sure, but I doubt that the rear valance was installed before paint.
It would mean that they would have to install it then take it back off to put the rear suspension (springs) in.
I can't see them wasting the man hours to do that having retired from a Chrysler Assembly plant.

It may have been painted at the same time, they did that with some parts suspended on brackets with the vehicle.
I know that's how they did it on the Ram trucks.