Being that it's cast Iron, I don't think welding is an option. Carefully drilling and tapping the bung may be an option and is easy enough to do. The port might be too far up the exhaust stream to effectively mount an O2 sensor, though. That being said, the port location is technically at a point where all four exhaust paths converge, which is what is needed. The best O2 location for that manifold & down pipe combo, would be at the collector where the flow become one. However this may be too far down the exhaust stream and may not be hot enough or provide accurate measurements. The overall design isn't really optimal for an O2 sensor, but that doesn't mean it can't work. I say drill and tap it and see. Worse case is it doesn't read or give skewed data. If this happens a simple block off plate will plug the hole. If it works, then that's great! Due to space constraints, may I suggest using one of the short style O2 sensors usually found on catalytic converters? I'm not certain if those are a higher heat range or not. Best to research that.