they turned out awsome!  i didn't have room for anything, cause of the super narrow frame.  i studied a bunch of mufflers a few years back for small air crafts.  then i helped build a street car that competed in the dynomax power to the wheels shootout and i got to see all there designs.  the shape of these mufflers have reason,  the 2 notches are there so that they clear the body mounts on the frame.  the stealth body shape was for clearance.  basically the main principle of a muffler is to slow down exhaust gases.  the more times you make it bounce inside the muffler, the quieter it will become.  the secret ingredient is 2 stoke muffler packing and perforated sheet metal.  use this and it will work out.  i have dual 3'' coming in off the collector and twin 2'' tips out the back.  i did this so that it wouldn't be so low pitched and drowney.  and it worked out amazingly.  here's a pic of the inside.  you need to in vision whats going to happen inside, how its going to flow.  keep in mind of what displacment yur motor is and run the proper size exhaust.  don't go to big for a smaller motor. match it up proper or you'll be choked