Take the car to a panel shop, along with a lot of money, and tell them to fix the rust and flare the guards. They should cut out the rust, weld in new panels than flare the rust free quarter. Will cost a lot though... unless if you are handy with a welder and bog, or know of someone.
Another couple of options is to get those fibreglass repair kits, and put more fibreglass onto them in order to pump them out... or buy another shell. Which isnt as crazy as it sounds
Re-skinning rear quarters FTL