20" will fit. If you want to lean more to quieter, dyno max 17747 (2 1/4), a little less quiet, 17748 (2 1/2). It is a bit unfortunate the way the exhaust has to pass over/around the exit tube on the "center" outlet muffler pipe. The Ultra flows avoid that, but they get a bit louder.

My observations indicate that you'll have less (maybe no) drone with headers with these mufflers.