on the stock exhaust system, put it just before the rear passenger o2 sensor in the downpipe, the sensor is not rated to be within 12" from the head, this puts it post cat on the stock system but ti is still the best place to put it IMO. you will want to get all the shavings out so they do not get stuck beating against the rear cat when you drive. you want the sensor angled to 9-10 o'clock position so that condensation does not puddle on the tip.