Put a new bolt through the existing nut? Or if there isn't a hole, drill straight through the "welded nut", and then put another nut on the end? This isn't aluminium, the size tools you'll be using on these steel fasteners will be nowhere near strong enough to do any damage. If god forbid you do ruin the threads, buy a new appropriately sized bolt/nut combination like Tom said lol.
Trust me lad, you haven't experienced pain until you've tried to undo rusted exhaust header bolts on a motorcycle. Steel into an aluminum head is a recipe for disaster, real danger to snap bolts/ruin threads which cannot be fixed easily. Quick fix with B&Q bolts is out of the question when you've got a stud snapped clean in the head.
Compared to the other task's you've tried to do, this one is pretty straight forward.