If the rust is confined to the floor pan you should be able to replace/patch the area. Not sure if new pan sections are available or if you will have to fabricate one. I know there have been some sheet metal and floor pan replacement suppliers discussed in the vintage forum. A search might prove fruitful.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.