HEADERS on a SCION TC?

all stock 05 Scion tC. I plan on an Injen CAI, Megan Racing Headers, Magnaflow Catback, and S-Pipe. Now, I am concerned with emissions when REPLACING THE HEADER.......If I replace the header do I have to have no cat in the header, or is there a way to install a new cat or old cat.....I AM NOT A CAR EXPERT, I AM TRYING TO LEARN, SO BE NICE.