what do i need to run a jdm itr header on a b16..i know that the header would not match up to the cat ...the header is too big for the cat...i would like to keep my cat=( thanks

 

You asked and answered your own question...

If you're using the 2.5" header collector you'll need a 2.5" flange on the cat. What you *should* do is get a new car because changing the header, only to add a bottleneck at the cat would be pointless.