Bodywork experts!

So I'm having an issue with my rear valance when I went to put it back on. The car just got both full quarters welded and when I checked the fit of my rear valance I noticed that the valance is now wider than the quarters by about 1/16th of an inch on each side to 1/8th inch at the top corners of the valance.

What's the best way to widen the lower quarters/trunk extensions to the correct fit? They were left unwelded at the rear pinch weld where the quarter panel meets the trunk extension but are welded along the bottom lip. :glasses7: