Thats naasty, I wouldnt even want to fix that. IMO no matter how much welding you do, its just gonna come back and rust on you. All the welding that is going to be formed is going to burnoff all the corrosion protection that is already there to burn off, which in turn, creates a hot spot for rust again. Also, your welds need to hold the entire rear in the car. Maybe I dont trust my welds enough to do something like that. I personally would get a new clean 4 cyl body and swap it up.