This is exactly what I'm doing, replacing my old and rusted centre and rear mufflers (450 SEL) with new ones for SE which I happen to have.
My question is: Does the 4 inch extention for the pipes need to be in front of or in the back of the center muffler. I havent been under the car to measure this.

Tomi

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MB 350 SL 2.8 -72
MB E220 -95
MB 230 E -82
MB 450 SEL -80 (project)