you should be able to remove the plastic inside the wheelwell and use a piece of screen as a backing for the filler. as far as the one in the frame goes I would need to look at it. I get a little nervous fixing structural pieces on a car. I would be tempted to weld a piece of 1/8th plate steel over it. But I will not swear by it. The best thing to do for holes in a frame is take it to a frame shop and ask them to do it. They have the experience and can tell you if it is going to be a problem. Hope this helps
