Judging by what 90Lan has said;

JDM cat = 30cm
AUDM ITR cat = 45cm

If as you mentioned, the exhaust (i'm assuming catback) is designed for USDM
market and is same length as AUDM catbacks, you will obviously still need to
add some piping to the end of the magnaflow cat since it is still shorter than
the AUDM cat.

I was assuming you were getting a JDM catback until i read the previous posts.
So with the USDM catback + magnaflow cat, the only modifications which you
should require is the triangle flange for the cat, the 2 bolt flange for the other
side of the cat connecting to headers and some piping on either end.