Right after the 2nd o2 sensor, because of my divorced wg downpipe. Excuse the crappy looking welds in my picture. The left sensor is the factory one, wb on the right.

The location in the other picture will have fast response, but you might overheat the sensor, and it will crap out sooner. Maybe try another foot further downstream, just remember to orient it towards the top of the pipe.

good luck

later, x