I developed a technique to fix the problem where a new reproduction rear valance appears too short to fit across the bottom of the car. Out of the box, there will be a 1/4” gap at the lower corners.  I have seen shops weld in new pieces or fill it with bondo, but none of that is necessary.  None of that is necessary. The problem is the valance contour is not proper, and they have too much curvature at the bottom. Even though the bottom edge is long enough, it is curved making it not fill the entire width.

Reforming the curvature of the top flange of the valance, where it does not show, forces the bottom of the valance to take a straighter line across the car, completely filling the corners.

Pics show how to clamp the flange into a bench vise, then bend the flange a few degrees so it for es the bottom of the valance farther under the car. This is all done by hand. Just bend it a few degrees all the way across the top flange. It will then fit perfectly.