You may want to stay away from a muffler with a center inlet or outlet, depending the muffler's design. A straight through muffler like a dynomax ultra flow, well you're fine there. A dynomax super turbo muffler like this one, if you follow the path of the exhaust you can see all the exhaust has to cross over and under the center tube at one point. Now if there's lots of room in the case, you're okay, but if there's not, you're giving up flow. Maybe not a big deal for a 318 but I would keep it in mind anyway.