Quarter Fitment Problem

If what your refering to is what I think I can see in picture #2, you should be able to drive the inner lip of the extension down (outward) with a good piece of hardwood or something hard and blunt. You may have to hammer and dolly the quater panel lip up slightly to make the contact that is needed. Then clamp the two together about an inch apart and work each weld separately. When it's all said and done you may have to tweak the two joined pieces back out or down in this case, to "square up" the lip.

Again really hard to give advice when I can't physically touch, feel and see it for my self. Hope this helps.