you know a big problem with the floor rust on these A and B body mopars besides a leaking windshield gasket is the wiper arm shafts.
the seal for the rotating wiper arm shaft to the cast pot metal housing dries out and lets water inside the car since the back half of this part is inside the cowl behind the dashboard. what i did with mine on both of my 2 old dodge charger restorations was to remove these parts from behind the dash when i took each car apart, drilled a hole in each of the housings in an angle that would be accessable when they were remounted, installed a grease fitting in each hole, and pumped a little grease in each one. this not only lubes up the shafts for the wipers, but makes a seal to keep the water out of the interior.
hope this helps
matt