I am not sure about a USDM ITR header...but you can get the JDM 4-1 header, a JDM OEM CAT or compatable high flow cat and the JDM exhaust and that will fit. Or atleast it will for the GSR. I have talked to the guys at King Motorsports a lot because I want to put the Mugen 4-2-1 header on my car but I also need the JDM ITR cat and the Mugen JDM ITR twin-loop exhaust. My only problem is the approx. $2000 to get the parts. But King has assured me it will all fit as they have done other GSRs the same way.