[DonC]

The O2 sensors for my mass flo are in the header pipes about 14 inches from the flange and mounted upright. About 20,000 miles on the system and no problems with the O2's yet. One of the things a narrow band sensor needs is heat to make it read correctly. Wide bands not so much because they have integrated heaters in the sensor. This is one of the reasons narrow bands don't read as efficiently when mounted in the collector in front of the muffler. Not hot enough.
DonC