The coarsest is generally best, but anything 24-80 will be fine. You're gonna destroy the panel you're grinding on of course and it will be easy to cut too deep and scar up the metal you need. In a junkyard situation I'd use the spot weld cutter bit I mentioned and something like this https://smile.amazon.com/dp/B000JFJMJ6/. I started out using a stiff metal putty knife but this is about 100x better.