If it is indeed a new engine it won't start very well until you get about an hour on it, by then it will have some serious compression and start with one backflip every time. A 16x6 is a little light for it. Once you get it tuned it will run very smooth. The muffler position should not be an issue unless it gets a lot of fuel in it. Set the LS needle at about 1/16 inch or slightly more from the end of the spraybar at full throttle. The Bisson Webra 1.20 /MDS 1.48 Sport muffler is a good choice for upright running.