a JDM header has a 2.5" collector (Anything that bolts to your stock cat is 2.25", not 1.75").
You do not want to cut up your 2.25" stock cat to fit a JDM header. It woudl be easier and likely just as cheap to buy a carsound cat made to the length you need. You can get them as cheap as $120. I am not a DC sports fan due to it's ground clearance. I prefer the OEM JDM 4-1 and an upper heatshield.
Trey