Rather than weld in a special bung, I might be able to get away with drilling out a hole and holding the bung in place using a muffler/exhaust pipe patch along these lines: https://www.caswellplating.com/
Prod 80333
Prod 80334
Prod 80335
Something like that looks VERY promising. And I could do it at home!
A bung like this https://www.diyautotune.com/product/innovate-motorsports-extended-o2-sensor-bung-3764/ or like this https://www.diyautotune.com/product/mild-steel-18×1-5-thread-bung-and-plug-kit-3735/ should be prefect for the O2 sensor.
Another option is a rivnut http://pc800archive.douglasvanbossuyt.com/message.php?message_id=100103
Still need to do research and select a proper wide-band O2 sensor… Looks like I’ll need to go for something along these lines: Yes, I’m willing to pay an extra $50 for a gauge to put on my dash 😛 Not sure if I want an analogue one or a digital one though. I’m favoring the analog gauge.