I did some more triage on the interior floor pans today - pictures at link.
Not looking too good on the passenger floors, but it appears limited to the middle of the panels, so welding in sections should not be too difficult. I even have some sections of the front left fender which I can use for some patches.
The "rear seat" area on the passenger side is also toast - you can see right through it. The area behind on driver's side is better and I should be able grind the surface rust off of that.
http://s284.photobucket.com/albums/ll4/pwogaman/1980%20Mercedes%20280SL/1980%20Mercedes%20280SL%20Disassembly/