Yeah, my car (the 82 300D) is suffering as well. Be sure and look over those floorboards real well. If you've had any water leaking, it will tend to pool inside the car on those floorboards and cause additional rot. Poke around with a screw driver to test structural integrity. I've sanded and POR15'd quite a bit, after every winter, but honestly there are nooks and crannies where it will hide and grow, and it has. Another thing I'm going to try before the nasty salty road season comes is to coat the bottom with an oil like
Fluid Film, it is lanolin based and supposed to help.
Good luck!
-Chuck