O2 sensor location?

I mounted mine about 8" downstream of the collector flange, mostly because it was a convenient spot. Would have preferred within 2-3".

Closer to the collector, or in the collector, is better. The farther you are from any one cylinder, the longer it takes for the sensor to get a 'read' and the tougher it is to keep them up to temperature - which is important for accuracy and to keep them clean.