exhaust help.

[noE30]

First off I have a 89 325i and I recently bought one of the chinese long tube header of ebay, I also have a IE catback. Now my question is what should I run as the midsection? I thought about just running a resonator instead of a cat, my problem is I can't seem to find any dual inlet/outlet resonators.
Is there one that you guys can recommend?

Also I read that the o2 sensor needs to be extended or you can use a 318is sensor, can anyone confirm that you can use the 318is sensor?

Thanks