Sorry if this is a dumb question.. but muffler/exhaust is not area.

My question to those who know is... would an aftermarket muffler that was designed for a particular new model, such as the new S-class or CLK, etc. work on a different model such as an older 560SEL? When the aftermarket companies "design" for a specific model, are they just designing so the muffler would fit or does the muffler itself is designed internally for that engine? I am thinking of purchasing an aftermarket muffler for my future 560, but am afraid there might be some sort of complication. I can have a muffler shop "design" to fit the muffler, but am worried if the muffler itself would cause a problem.