Thanks for the advice, folks.

It never occurred to me that perhaps the rockers could add anything structurally. The raw replacement panels seem quite flimsy and the actual frame is hidden beneath, so one could be excused for ignoring what impact, if any, they have on rigidity.

I'll talk it over again with the body dude.

I'd not have dwelled on it if this were a '77 or newer 240. But given the scarcity of W115 parts now, I shudder to think at the effort and expense of sourcing body panels in 15-20 years. I'd like to do the best thing now to extend this quasi restore out to benefit not only myself, but any future owners as well.

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1974 240D "Boldie" 170K.- New timing chain/freshly rebuilt IP/replaced valve seals/injectors/upgraded stereo/new Bilsteins with Yokohamas/fresh paint and rocker panels plus lots of welds.