[DonC]

It sounds like you're looking at a system that uses WB 02 sensors. The primary difference between WB and NB sensors is that WB have their own internal heaters and can be mounted just about anywhere in the exhaust stream with the primary consideration being a position that gives you the best access to the entire exhaust stream. NB sensors fall into the 18-20 inch from the port range to get them hot enough to read accurately.
Mounting the sensor at the horizontal or above is to keep condensation from gathering on the tip of the sensor and corroding it.
The common problem with our cars is that, in general, the collector is so close to the body there isn't room to install the sensor where it will be hidden from view. About the only way to get the necessary vertical range placement is to cut a port in the body behind the collector that allows the sensor to protrude inside the body. Not an ideal location but that's about all you've got to work with. Short of that your only other option is to mount the sensor below the horizontal but be prepared to replace it every so often when it gets fouled.
DonC