DC sports is junk. Sorry, but I had to say it. h:

The stock cat will have to have a 2.5" flange welded onto it to accomodate the 2.5" collector of the JDM ITR header. You could upgrade to a highflow cat that has a 2.5" flange already welded on and the stock sized flange welded on the other side if you want to avoid a muffler shop.