From searching, it seems like a general guideline for wideband AFR sensor location is that it should be after the turbo to avoid exposing the sensor to the high exhaust temps before the turbo. Another general guideline is that you should avoid mounting it too far downstream because it takes time for exhaust to travel through the pipes, and you want as little lag as possible between exhaust exiting the engine and causing a reading on the sensor.

If I were to add a AFR sensor, I would probably put it on the downpipe, after the downward bend, so that the wiring could be routed up between the alternator and throttle body. Basically, somewhere on the part of the downpipe that is visible from above, to the right of the downward bend that is right in front of the alternator:




Just find a spot that fits well, lets you route the wiring nicely, and don't mount it horizontal or below horizontal: