side exit exhaust

You can use a more normal muffler with with Offset inlet and a center outlet under the backseat,then use a 180 or 2x90degree bends with appropriate radiuus to suit the spacelimitaions you haveto route the tubing forward between the mufflerbody and framerail,,then ad a fairly shortradius 135degree bend to go around the front of framerail out under the rockerpanel.

Hope that was written in a way that made some kind of sence.
Notice that following this approach works best if you fit your mufflers as high as possible under the rear seat(no it should not hit,leave adequate clearance just like with everything else) and you are also alitle limited on how big mufflerbody you can use but it works well.