The aftermarket supplier will send the correct flange with the kit. It is a very long cat back. I have the Magnaflow cat back for my 15 2.0 SX-T and I love it. It actually goes all the way up to the back of the engine. It connects to the midpipe. somewhere up there.
I can pick up another 20 hp if I bought a new mid pipe with a catalytic converter. that way plus my Lap3with 22lbs of boost instead of factory 17lbs. ....but no matter what you do you will LOVE the sound of a new CAT-BACK. GOOD LUCK MAN!!!