Best spot to mount wideband o2 sensor

[art leong]

I have one of mine in the collector only 9 inches from the end. It is useless till around 4,000 then it works fine.
I have another in the primary tube (number 4). They don't read the same. I'm going to recalibrate them both. But I want to do it at the track. So I can see if there are any changes in the numbers and what seems to be the optimum number for me.
Right now the car runs quickest at 13.5 to 1 AFR. I don't want to recalibate them in the garage and have that number change. And be lost when trying to tune at the track.