Honestly,to me it looks like the quarter doesn't have enough curve to it....AND the valance looks like it has too much of a curve.
Before you start messing with the quarters, I think it would be a good idea to test fit a different rear valance. That rear valence has been modified with those extra holes...you really don't know what else has been done to it.
If you want, I can go out and measure my car, side-to-side where the valance meets the quarters and see where we are at.
OK...I went out and measured my car just to give you a reference. I held the tape measure against the bottom flange of the quarter panel where it meets the valance. I measured straight across to the other side and got 4' 11 1/4" +/-