I think that if you get an ITR LSD from a US Spec ITR, then it should "bolt" right in, as both the ITR and GS-R (US spec) have the same final gear ratio (4.400).

Should be a relatively bolt in procedure, and any decent tranny shop should have no problem dropping it in...

Though, I dunno about the J-spec ITR, because I know that has a different final drive gear....