I've rebuilt at least 2 122's with box channel/floor rust that bad. It's not terribly difficult if you have proper cutting tools (preferably air powered, as they take up less space than electic tools), and are reasonably proficient at welding with a MIG or TIG machine. I fabricated the frame box channel sections out of what I think was 16 gauge sheet metal. (memory is a bit fuzzy on that) For the floor I used sections of sheet metal that I cut out of the roofs of Volvo wagons that I sent to the "great beyond." I also now have a bead roller that I got from the Eastwood company to put the stiffening ribs in the floor panels, though, you can get by without it. Just make sure that the inner rockers aren't rusted out as well, as you won't have anything left to attach the floor sections to. If you can get proper patch panels, so much the better. It will make the job go much quicker. Whatever you do, do NOT use lap joints if at all possible. I prefer to use butt seams to join panels, as it's much more corrosion resistant. I also recommend using "Rust Encapsulator" from Eastwood to seal the insides and the surface of the metal. That stuff is pretty impervious when applied correctly.

-Nick