I have a flowmaster dual 2.25 in/out muffler and it sounds great. I did not run an x-pipe since the muffler is an open chamber and the gases mix in the muffler and then split on the way out the exhuast. They make one larger for big blocks. I don't know performance-wise if it made a difference but I am running 2.25" pipes from the cats thru the muffler and standard duals out the rear of the truck. I think I spent $180 buying the muffler, tail pipes and two 5 foot sticks of 2.25" exhuast pipe to run to the muffler. I don't know if I would do it this way again but I am happy with the results.