Stock is just about 2.5". Go for the 3", it can't hurt anything and should help EGTs and let your turbo spool a bit faster.

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine