All of the JDM headers(Toda, Mugen, OEM JDM 4-1, JDM DC, etc) have a 2.5" collector, and will not bolt up to your stock cat. You will need a 2.5" cat, and have a flange put on it to match the JDM header if you choose to run one. You can have this done at a muffler shop, or purchase a cat pre-made to fit JDM headers:
http://store.hybridgarage.com/summar...tegory=Exhaust
http://tech.hybridgarage.com/tech/jdmhdr/index.html