If you are concerned with the outer edge of the shock mount, the part you would put ther upper bolt through first, that is common and easily fixed. Ideally you should weld a new piece of metal in there but a riveted patch will work too. It is not a highly stressed area but guides and supports the upper bolt. The inside frame rail is where the bolts screws into a fixed nut. If that area is rotted you need strip the interior and open that area of the floorpan to weld new pieces in. It's a lot of fabrication and that's why, if available, you would cut the corresponding piece out of a donor car and then weld the entire piece in.