the reason those cars 83,84,85,86,87 240s rust out the floorpans is from inside out.either windshield leaks,roof rack,am/fm ant.i live in south and have looked atplenty of so called creampuff 240s only to look under and laugh.unless you are half a structural engineer,the best way to fix floorpans is to cut or treat the metal the best you can[watch out around center support bearing]weld metal back into place and fiberglass over top of that,then unmdercoat from below.i realize that this is kind of vague so if you want i can email you with more precise ways of doing it,i have fixed several 240s.