Usually when installing a new rear valance, the lower outer corners will not reach all the way to the quarter panel corners, leaving a 1/4” to 1/2” gap.  Most assume the valance is made too short. But, the problem is the valance has too much curvature across the bottom.  There is “x” distance between the lower points of the quarters. The valance needs to extend “x” distance between those two points.  If you measure across the bottom of the valance, following the curvature of the metal, it will also measure “x”.

The problem is the distance between the quarters is measured in a straight line. Taking the “x” dimension and making it follow a curve means the straight line distance across the valance will be too short, hence the crappy fit.

So, you have two options to get that curved piece to fit across a straight line.  First, you could take a BFH and pound it flat on a 2x6 and spend a week applying bondo and sanding to get it straight again.  Or equally painful, weld in some extensions at the corners.

Or, the MustangSteve “work smarter, not harder” method.  Here is how to fix it simply.  There is a mounting flange at the top of the valance with 11 attaching screw holes. Take that flange and clamp it in a vise a few inches at a time, and bend the flange to more towards the front of the car. In my simple diagram of a cross-section of the valance, take the solid vertical line representing that flange and bend it to approximately where the dashed line is. Bend it maybe ten degrees or so.  This will effectively force the BOTTOM of the valance farther forward as you tighten the top screws. Pushing it forward flattens the curvature of the bottom edge, making it straighter so it reaches all the way from corner to corner where it meets the quarters.   Magically, the valance will now fit correctly.  Since the flange you worked on is hidden behind the rear bumper, any booboos you introduce with a hammer will not show.

Yes, this really does work.  I figured this out about thirty years ago. Hopefully, rear valance manufacturing has improved over time and your new one will fit flawlessly right out of the box.  But, if not, this is how to cure the fitment issues.