I am wanting to swap the stock muffler. Had decided to go with the Donaldson 40" M085171, called a buddy that works for freightliner and he checked the #...it is on national backorder. I called Truck Pro and they said it was discontinued. My buddy checked his parts room and said they had a 3.5/3.5 center straight through with a 30" body length with about a 9" round body. That would bolt up to my 04 since it is 3.5 from the turbo to muffler. The stock muffler is 3.5/3.5 with a 4 tailpipe so I may try this one and see how it sounds. I can test it for free and even though it isn't 4/4 it has got to be better than the stock garbage can.