i had the 65 style shelby side exhaust from tony branda.....glasspack style mufflers and all the piping for like...$300...it sounded like a harley until i cut off the sidepipes...there was too much backpressure(2 1/4" dia). they sounded real nice...resonated ALOT, and screamed at 6000 rpm. its loud because its by your ear...and it if it resonates at all...you will definitely notice it. when i get another 67 im going to figure out how to run side exit with midas mufflers...the meanest mufflers ive ever heard.