That is not an easy fix. And sadly I doubt that is all there is to fix. As rotted as that looks, there will be more. Check the floorboards from underneath, check the bottom of the rear quarters, check the bottoms of the doors, check the cowl (up under the dash). Also it looks like that car has a vinyl roof, you can almost bet half of the metal under that vinyl looks like the hole on the back deck. I hate to rain on a parade, I just don't want to see you get yourself in over your head and find that you need to spend thousands on metal work.