I bought a mig and some repair panels from Klokerholm and did the rust repairs on my 81. IN both sides I had simialr to what you are describing except mine was in both the rocker panel and the floor panel. Especially around the seat brackets. Once you get past the learning cruve of mig welding very thin sheet metal as long as you have the right tools (body hamers and anvils, angle grinder, welding clamps, etc) it was fairly straight forward. Getting the hang of making good butt joints is 3/4 of the battle. My floors are super solid now.

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1978 Mercedes 240D
1981 Mercedes 240D
1982 Mercedes 240D