catalytic converter help

ok..as many of you d16y7 y know..the exhaust manifold has a built in cat. right? ok mine was cracked when i got my car. so i opted for a header. an ex one. and it came w/ a place for the o2. and i have 2 o2s. so i was thinking of getting an ex cat. and cut off the triangle flange and just weld it to my existing pipe. but i need an o2 after the cat. i think the 99-00 sis have an o2 on the cat itself. can anybody reassure me or can recommend a cat that does (that will bolt up to the ex header of course) anyones help will be greatly appreciated