Inner rocker panel is best to rebuild using scrap sheet metal. Also, if it is that far in, chances are part of the floor is gone, too. Take a look at the rear strut towers carefully inside the trunk. They're probably starting to pop through. As well has look for random buldges and rust stains in the front wheelwells near the frame. Ask me how I know all this. If you need examples, search IncREDi-hoopdy.