I`d cut the bumperbrackets and weld them where you want the bumper to seat and go from there. In the end you want to have straight gaps and if you have to cut the valance 20x to get it right ,so be it, but if your bumper sits crooked in there, it will never look straight.
But the real problem is that the valance is to long and doesn´t fit at all
The quater patch panels are changed but the upper hole for the valance IS NOT moved and that is also in the wrong place
If I shall fix that I have to move the whole rear fenders out to get a wider body but the the bumper will have to get wider also
So I make a new valance in fiber glass that fits on my car