I would put the downpipe on the car, get under it and pick out a spot.
On my THDP it's past the last bend facing up and towards the engine. It was so close to the dump I bought a stainless band clamp to make sure it didn't get fresh air at low CFMs.
The last thing you want to do is weld the bung and have it put the sensor into sheet metal, where it can be changed, or where the header burns the wiring off.