MAC is awell know mod purchase site
http://www.macperformance.com/store/...&Category_ID=1
Go here ans locate your mustang year, etc. and look at exhaust options. They also have frame stiffeners and much more.
There are many site but I like MAC and they are well known for their exhausts and are said to be right up there with Flowmaster and others. I hear they have a great sound.
For the V6 you buy the "cat" back exhaust system and a catted H pipe set up. Cat means catalytic convertor. So if you want horse power you can not just get an exhaust systen that ties in to the standard y pipe and gives you two exhaust pipes and two mufflers since that does not give you the horse power that taking the sytem all the way back to the manifolds does. Just adding on to the existing y pipe is not true dual exhaust. To get the added ability to remove more exhaust gas thus adding the horse power you have to remove the restriction created by the standard y pipe design where both sides of the engine exhaust are combined after the manifolds. The H pipe also "tunes" the back pressure. An X pipe does something similar but hear they make a droning sound on the hiway and H pipe is the better way to go.
You will also see "off road" H pipes. These are typically put in while removing the catalytic convertors not street legal in most states. Gives more power and better sound but not legal where environmental controls are required except for racing.
Look around the site and you will see both set ups. 1 keeping the y pipe and 2 replacing the y pipe with true duals
Hope this helps