When my Dad's 71 280SE got repainted decades ago, one of the bad rust spots was the rear wheelwell...you could look at the pavement from the trunk if you peeled off the rubber trunk mat!
The bodyshop fix was welding a thick slab of galvanized sheetmetal onto the body and further "rustproofing" with roofing tar! To this day, it's still solid!
Not a "concours" way of doing it, but this was a driver at the time, and the car was already heavily undercoated to protect it from the salty roads during those harsh Illinois winters...
Back then, my trusted mechanic showed me a 280Z where the floorboard was nonexistent...so he riveted road signs onto the good metal...imagine what the other mechanics would see when they put that car on lifts!:p