I would try a couple of things:
Heat the casting, apply a little beeswax. Let it soak for a few moments.
Or, heat the casting, use some PB B'laster. Positively the best penetrating oil ever made. (NAPA, or AutoZone carries it where I live)
Or, heat the casting, use Loctite Freez-Away cold spray on the stud. Probably less heat than you'd think, I frequently use a small pencil-sized butane torch.
The suggestion to alternately tighten and then loosen is a good one. I've pulled many stuck studs from castings, and never needed anything beyond a double nut and these techniques. Heat and/or cold will definitely help!
1934 South Bend 9" lathe, Tradesman 16 speed drill press, Grizzly 4x6" band saw. (Sad, I know...)