The "drip rail" is held on with screws that are stainless steel. They screw through the drip rail into plastic "keepers" pressed into the sheet metal. The rust is almost surely only around where these "keepers" press into the holes in the body sheet metal. I would get a few of the plastic keepers (dealer only, but cheap) ahead of time and proceed as Peter suggested.
The Golden Rule
1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)