Thats a common problem with these cars.. as far as patch panels,,, there are a number of sources, and depending on the amount of problem area you have, you may be able to form your own out of 20 gauge sheet. If you want prefabbed panels you may want to look at
http://www.shermanparts.com/ or
http://www.autobodyspecialt.com/ As far as the underside support, well, you'll see that the lower section of the quarter is connected to the trunk extensions, and if you dont keep the drain cleaned out it will give you problems over time. AS far as cutting them free,,,, NO!! if you do that there will be no lower support on your quarters and if you lool down the inside of your trunk,,, I dont think you will like looking at the ground below. Hopefull you wont have to replace the extensions before you do remove the drain plug and try blowing all the dirt, crap, lost lugnuts and God only knows what else that has accumulated there over time Then flush it out ood with a hose. It's rusted now so a little more water isnt going to hurt it. A quick fix, after cleaning it out you could always flood it with por 15 but,,,,,,,,,,,,, I'm gonna get bashed for that suggestion. it will only be a temporary fix. Either way good luck and happy patching