Don't buy from Amazon, the tune they ship you is for a stock car. Buy from one of the many Mustang Vendors mentioned here and that advertise here. Tell them what you have (Headers) and they ship it with the proper tune. And most include "tunes for life", so if you add more later, buy from they, they ship the part and email a new tune. Which tuner supplier to use is the classic Ford Chevy argument, no right answer for everyone. Pick a vendor used here and chances are in your favor you will be happy.