JFPufNStuf,
my car was in similar condition. Repairing the sills is major surgery and can take several months (8 in my case) and many buckets of dollars.
I also know of another car with the same story, but the owner had an interesting repair made that greatly simplified the work. The sills are roughly triangular in cross-section, with two of the points flattened to form the flat bottom and the flat outer vertical side. It's the bottom that rots out, but the remaining metal may be sound. The simplified repair involved cutting out the bottom and welding in a new section to repair it. This avoids having to remove and replace the entire sill structure. Had I known, I would have tried this approach, too. Depending on what you want from the car, and the condition of the sills, it might be worth looking into this type of repair instead of the major surgery.
It's also possible that the box-section stiffener in the front fenders is bad. It's inside the fender on the engine bay side and can be seen by looking in and up; it runs from the headlight bucket to the firewall. The flat top rots, because that's where crud and salt collect. You can't see the top, you have to feel it; but wear gloves, because if it is rusty you'll shred your fingers. The book method for replacing this stiffener involves removing the fenders, because there is no way to access the top to weld a new one in place. Removing the fenders is another major surgery. But a new stiffener can be installed by splitting it lengthwise, welding the upper section in place from below (inside the stiffener), then welding the lower section in, and then welding the two sections together along the split.
Hope that's somewhat useful info, and good luck!
George Davis
'69 280 SL Euro manual
