If you want the side exhaust... go for it. Like everyone has said it won't gain much if anything but it is a neat appearance and sound modification. I definately didn't lose any hp. As far as clearance I have had no problems. I've been over speed bumps, potholes, etc. and have not scraped anything yet (of course it is lower so you do have to use common sense.) My setup is the Borla kit that has mandrel bent pipes throughout. The Classic Design Concepts kit is a good kit with I believe Dynomax mufflers? There are tons of Stangs around but not too many have side exhaust so it tends to be a head turner. The only drawback is that the kits are expensive and that translates to money that could be spent on other go fast goodies. I got mine for half price from a guy that had it on his car for about two weeks and then went a different direction with his mods. Just be sure to follow through with a hi-flow h-pipe and you'll be on the right track.
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66 Tahoe Turquoise Coupe
Laser Red 96GT Ragtop
30 Years & 30 inches apart in the driveway!