[Bruce Edwards]

O2 sensors work by heat. They make their own electricity by way of the heat exciting the material inside the sensor. The gauge reads this as rich or lean by using a standard voltage for the correct mix. The placement of the sensor is important in order to get the correct reading. To close to the head and it will always show lean. To far and it will always show rich. If you look at most new cars the sensor is placed just before the pipe hook up or shortly after. A secondary is used after the converter to be sure it is working. The secondary will always show a lean mix due to the heat of the converter. If you pull the converter it throws off the sensor and kicks you check light.
So if you are installing a sensor to check your mix you will want it placed were all 4 pipes come together in the collector.
I see no way to install one with out welding a bung to the pipe. You could weld it to te bottom so it can not be seen and plug it when you are not using it.
You could also check your instruction sheet to see what it says.