I had a shop install my O2 sensors. I have headers, Doug's electronic cutouts w/a Y that the open end just dumps the exhaust out under the car (no pipe added to the open "dump" side of the cutout) The exhaust system side is all 3" pipe, AND I have a prefabbed X crossover in the exhaust.
To answer your question, my sensors are at 3 o'clock on the driver's side and 9 o'clock on the passenger side. I have the Innovate Motorsports DLG-1 DUAL Wideband O2 sensor system with the included gauge that reads out BOTH channels at the same time...
I got it for tuning my 6bbl, and to monitor the A/F Ratio under all running conditions.
Example: I had read about the "lean spike on tip in" of the outboard carbs. There is a lean condition right when the outboard carbs open, but in my case it lasts less than 1 second. It spikes from say 12.8-13 to 1 up to 19 to 1 lean, then goes back to normal. That is vital data, for instance, if you use Nitrous (I plan to) you would want a 1 second DELAY once the WOT switch is closed before the Nitrous sprays or you will be spraying the center carb right when the outboard carbs go lean!
Anyway, I also have the brand new Dakota Digital RTX series gauge system, and that adds a "remote dash" capability for critical data (oil pressure, water temp, RPMs, etc) to be selected to display on a smartphone or tablet in real time. I bought 2 Dakota Digital BIM-13-2 modules that connect to the analog output of the Innovate system. That will allow the RTX gauge system to display the left and right side A/F ratio in the data window at the 6 o'clock position of the speedometer, so I could hide the Innovate gauge display if I wanted to, BUT MORE IMPORTANTLY I can get the A/F data on my phone or tablet while I'm under the hood. Unfortunately the Innovate gauge is NOT waterproof, so it can't go under the hood, thus the reason why I got the Dakota Digital modules.