If you're going true dual, it wont really matter WHAT you go with, the results will be the same.
The 4.0l v6 mustang sees gains from 10-15rwhp from just an aftermarket muffler in the single exhaust configuration. Changing out to duals honestly wont give you much more hp (1-2 at the most) if any at all. Just something to think about.
But to answer your question, i'd go with a Pypes H-pipe, midpipe, overaxle pipe system with dual Flowmaster American Thunder mufflers (the mustang GT Flowmaster A/T kit) :gringreen
Someone smacked me in the rear of my 2005. I got $700 from the their insurance company. I bought a GT takeoff rear bumper with takeoff GT mufflers. I found a stainless steel Y pipe conversion kit. Did the whole thing myself with $150 left over. Looks great, sounds great. The GT rear end is way sexyer!
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