Chris,
The wideband needs to be a certain distance away from the exhaust chamber to keep from burning out so use that as your distance gauge of where to mount it. In my case it got placed just in front of the center resonator since I don't have hany cats to worry about and it needed to be further back due to higher exhaust gas temps on my car.
As far as getting a bung, if it didn't come with one (it should have) any exhaust or parts store should have one. Just take the sensor in with you in case it is some oddball size though I think they're mostly standardized.
Rick
