Floor pans only rust from the inside if it gets wet. Have you found where the water is getting in on yours? My 83 (sold) had persistent water on passenger floor when it rains. Most of that water came in from under the battery tray, where the sheet metal (about 3" diameter area) rusted through.

My pristine 85 with very little rust has slight surface rust developing under the battery tray. I am cleaning that up now including the battery tray.

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked