Spark plug is M18 X 1.5 (the big one) and the small plug is M14 and some of the new plugs are M12 threads.
That is not the normal M18 O2 bung that is easy to plug with a spark plug.
The 2.5 pitch makes it a hard one to deal with. If it were the more common 1.5 pitch it would be a lot easier. But I would get that speciality bolt and plug it before I welded it up. If you still have the EGR riser that screws into that, pinch it shut and weld the pinch then screw it in to plug the hole.