Hey, I just want to clear this up for myself.. don't hate haha

I am about to buy a JDM mugen twin loop muffler with a JDM B-Pipe (because i need the length).. It essentially turns into a JDM catback... However.. i still have a USDM Cat and header (which will be changed to a JDM header as soon as i find one at a decent price..... anyone? shipping to canada? haha)

I was wondering what I need in order to make this set up work.. I know the JDM cat is 12-13" or something.. where the USDM is 16"ish? .. would I be able to get a Cat or test pipe that is 12" and it will all bolt up? or are there bolting differences? or a header length difference?

Just want to know what I'm getting myself into haha

Thanks!!