Buy a nice fat _round_ one , the longest one you can find so it'll muffle the sound well without reducing power . moving it back as far as possible wil give you a little bit of extra power -but- the trade off is slightly shorter muffler life .
Me , I don't run any muffler at all on my 235 and no one has _ever_ noticed , even when I pass through Police roadblocks at cruising events....
The 216/235/261 is so softly cammed that it runs quietly , almost a musical tone to the exuahst as long as the pipe is long , all the way to the back bumper .
-Nate