A word of advice....buy nothing for a Mustang if it’s sold by a company that specializes in "Sport Compact" performance. More often than not, they're just cheap knock off kits converted to "fit" a Mustang and most of their dealers know less about a car with more than 4-cylinders than you would. I was going to order a set of 2 1/2 so called “high flow” cats from a place that specialized in imports exhausts on Ebay. (2 ½ inch is 2 ½, right?) I told them they were for a 5.0 Mustang and he told me that not only would he not combine the shipping and that I would have to pay to ship them separately, but that he didn't think it would be a good idea to run them on a big block engine like that!?!?! Needless to say, I looked else ware. Long story short...if it sounds too good to be true, it probably is. Stick with a place that specializes in Mustangs, or at the very least Domestic Vehicles when you buy your parts.
Be aware they DON'T come with instructions, even tho the install is pretty simple....the one i bought off ebay for $33.00 works great, i could feel the dif.