You want to weld on the bung on the downstream side of the collector so it would be near 6, 7, 8. From your picture, I'm guessing that's the driver side header. You will need to weld them between the 2 and 3 o'clock position because space is tight below the car (so opposite side from 9, 10, and 11). If you weld them any higher than that, there is a good chance you won't be able to get your O2 sensor in the bung.
I attached a picture of where I had mine welded on. I actually welded mine on the exhaust pipe because there was minimal room on my headers. Ignore my ghetto heatsink...
I would recommend putting one bung on each side so that you can monitor each engine bank.
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