JDM and AUDM catback is the same LENGTH. Not too sure if the AUDM cat will fit the J's B-pipe, unless you cut the AUDM cat and flange it so that it fits the Catback. But yes, you cannot run JDM cat with AUDM header. The AUDM header outlet is tiny compared to say a Toda header outlet.
Sorry guys but reading into forums i get alot of people sayubg AUS headers does not fit JDM cat..