A really simple muffler to modify,,, Just remove the muffler,,,,
take the cover plate to the grinder grind back the opening to a point that seems to be 80% more than what you started with,,,, add a couple of 1/4" holes that will fit in the recess pocket under the cover is over,,,, triple rinse with solvent to get all the shavings out and reinstall you most likely should remove the limiter caps and retune the carb using this method
http://web.archive.org/web/20051018212959/www.madsens1.com/sawtune.htm
or if you really want to go dual port with pipes some one did a thread with walkerized style muffler on a 310 which will work see this thread
http://www.arboristsite.com/showthread.php?t=59958&highlight=ms290+muffler+mod
he actually blanked off the plate that makes the original cover exhaust outlet and added dual pipes above it,, to do it you must be proficient at welding/brazing !!!!! send the thread starter a PM I bet he will walk you through what he did!!!! I need some rep BTW!!!!! for all this research!!!!!!!