I once, about twenty-five years ago, drove a 123 280e. The exhaust system was shot from one end to the next.

I was able to find a 280e in a wrecking yard and pulled out the entire exhaust system in one piece. There were two down pipes coming off the M110 engine. I don't remember if the dual pipe arraignment continued throughout the system.

But I do know that the 'center muffler' was actually a catalytic convertor. It just looked like a muffler. The rear resonator was the only true muffler.

And I think this is why you are running into the 'non compliant' situation with CARB. I don't know if there is an exemption for cars after so many years (in Oklahoma any car over 15 years of age can have the cats removed) but you might look into that.

Or you might consider looking for a wrecked 123 280e in a salvage yard and just pulling the entire system. It would be a bit easier to replace it all if you can jack the car up high enough to twist it under there.

I cut the two pipes just in front of the end of the front fender on the right side. There is very little clearance in there so this was harder than it sounds.

You might also look for a 116 280se. I know the exhaust system itself would not be the same but the center muffler might be.