hey man welcome! former DSM guy here also. i know what you mean about the swarm of bees exhaust note the dsms have.... drove me nuts to. anyhoo i just put a MAC catback on my 99GT and it sounds pretty good to me not 2 loud but not stock sounding either it has a very low tone. maybe this link will help you decide on what brand you wanna go with as there are so many out there
http://www.v8mustangs.com/TechExhaust.html
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1999 mustang GT
2.5 MAC Cat-back
K&N filter