This is one of my favourite ways to tackle seized fixings. The extreme heat of the welding process heats up the fixing which then contracts on cooling which all helps to loosen it too.Would the weld have to be professional 9r could I just tack it at the sides were top nut sits
"professional"? well, you do need to be good enough to get full penetration with your weld so It's probably not a job to attempt if you're not reasonably proficient. Doesn't need to be to North Sea Pipe Welder's standards though.
The technique if you have studs and nuts is a bit different, so won't waste time discussing here.