I know that the X-pipe of choice is the Magnaflow 93335, but searching the web for prices I ran into this pipe @ uprproducts.com. It says its for the 2004 Mustang GT/Cobra. Aren't the DOHC engines wider than the SOHC?:
Link here: http://www.uprproducts.com/shopping/shopexd.asp?id=731
It doesn't say whether it's SS or not, but it's less expensive than the Magnaflow. It's also shaped a little different coming out of the headers. Is there any reason why this one wouldn't work in the same way the Magnaflow does?