OBX headers require major modification to work.
Skip them and get the SSAC's
and yes, it'll still work on the 04+ cars.
If you dont have testing, and aren't worried about the fine, don't bother buying a cat, just use the pipe they come with.
If you're concerned about it, get a 3" I/O high flow cat.
theres only a small sound difference between cat and no cat so I wouldn't really worry about it.