+1 for the "jig" described above!
DON'T use Easyouts (Described to me as "The most misnamed tool in the workshop" )
Many years ago, someone removed the head from a Triumph Mayflower, where, because of moisture and different metals, the studs had corroded the head. He made up a close fitting, thin tube to fit over the stud, with teeth non the end. This trepanned the rust so that the head could be removed.
Sometimes, a stud can be removed after hammering on the end of the stud, to break the rust, but not always.
If the stud can be rotated, even slightly, penetrating oil, and wriggling to and from may enable the stud to be freed enough to be removed.
Howard