The inner panel needs repair/replacing as well, so cut out enough to allow access for panishing. Purpose-built dollies may help for access, such as the U-shaped one below, another has a long arm on it, but I don't believe I had a picture of that one. Once outside is installed and planishing complete, for welding in the inner panel the distortion will be less of an issue as with the outside. But that panel should also be fairly straight bends where you may be able to fabricate the inner more readily as a one-piece replacement, for spot/plug welding back to original mounting flanges.