Itr collector size

i have looked for frikken ever to find this out...maybe i just havn't checked in the right places, but i heard that the header collector from itr's is 2.5 inches, but people also say that the header, cat, and catback from an ls and an itr are all interchangeable. now if the header from an ls is 2", but an itr is 2.5" this would be impossible...so....wtf? i just bought an itr 4-2-1 header and i'm trying to find out what exhaust will fit.

i really just want to know... will my 4-2-1 itr header fit any year itr catback? also if i choose to buy an aftermarker catback, should i look for one that says it fits a type r? (this is all going on my 2-door, 94ls)