You need the proper tool for removing the exhaust flange nut. When using the correct tool if it starts to feel stuck you should feel it and at that point cut the nut off. These threads are to be coated with anti-seize. Many recommend other brands. I use the Permatex, silver one.
The best repair does seem to be welding new Aluminum and then cutting new threads on the head. This is a really big thread and the die for cutting it is probably not too common. I see the die for sale on Ebay sometimes and it is a $100 I think plus shipping from Germany. I hope you can do this. Maybe there is some other way to cut new threads or the die is not uncommon like I suspect.
It does look like that head has been repaired before.
Looks of the two part flange repair aside it does work.