I think what's he's saying is that the weld is stopping the nut from rotating, no matter what the socket size is. Look like it was all bolted up then the weld added later by someone not scared of a long MIG wire In which case your only option is to cut the old not off - I have an oscillating saw that would do it - where about's are you?
Yep I think the welding was done after, I had a go with the dremel quickly last night and that seemed to work ok, it will be slow going as all four nuts are fouling the welding funny that the other side hasn't been done like this... Cheers for the offer btw