Here are some pics of my down exhaust on my M8540HD12 (ROPS). The stock stack on the Ms go straight up where mine is going down. This is for the ROPS model. On the CAB model, the exhaust shoots out the top of the right side and straight up the front right pillar.
As you can see, my down exhaust (and therefore the stock stack on the ROPS model) is on the left side.
If the exhaust on your L is like mine, the M stack should work or be workable.
My muffler is sitting cross-ways on top on my engine all the way to the rear of the engine compartment. So the exhaust comes out of the turbo on the left rear of the engine, up to the muffler that is real close, in the left end of the muffler, through the muffler and back out the same left end of the muffler, then down to the pipe you see in the pictures. You can see the turbo in the last pic.