Funny I just started to tackle this area myself this afternoon .... on my Lancia Fulvia. Same spot rust, a bit larger. You can see where I started welding in the first repair piece I made. Once the front is done, I will make a floor section complete with ridges, from 19 gauge steel.
So if I can do something like this, this being my first ever experience welding and fabricating repair panels, so should any body shop ....