Most of us use the 1.5" O.D. headpipes. True, they're not 1.5" on the inside but they are larger than the stock ones so they flow better. Mate those to the 1st muffler in your list, the 1.5" inlet Commando. This original Commando sounds wonderful, a little louder than stock but not too loud. Best sounding muffler I've found so far.