I have this header on my 89, when I bought it the guy on the phone told me to place the O2 sensor in the port just off the head in the number 2 primary tube.

Seams to me this would only tune for the number 2 cylinder, ignoring the rest? Should I move the O2 sensor to the bung in the collector, or leave it in the number 2 primary bung?

 

I will assume you have a Narrow Band Oxygen Sensor. Mount the sensor about 2" from the exit flange of the exhaust collector in the 12-1-2-3 or 9-10-11 O'clock Position. Do not mount the sensor in other positions due to moisture and / or contamination. If you are running a Wide Band Oxygen Sensor mount the sensor at least 6"-8" from the exit flange of the exhaust collector in anyone of the previously mentioned positions.