If you can get the extractor I'd do that ...if not tach weld a nut to it.....I would put the nut on and weld the center of it up, don't weld on the outside of the stud you could undercut it and make it smaller ,causing it to twist off when you try and unscrew the stud.
ALSO,Many might disagree, but As long as you are careful and avoid getting it on the paint....one of the best things I've ever used to get a rusty bolt loose is BRAKE FLUID. PB Blaster is what I normally use but have used brake fluid many times while in a junkyard..JMO