Soak it down pretty good with PB Blaster. Leave it sit a couple nites with the stuff. Tapping on the bolt can help it get in there good. heat wouldn't hurt (On the bolt), but be careful. Then, yeah, listen to Tony, grind it down flat, Not flush with the intake, centerpunch and drill it, use a GOOD extractor, should come out fine. if it doesn't, weld a nut to it and try it all over again.
Just the way I would do it. I'm from the east coast, so EVERY bolt did this back there, lol.
Probably why I still put never seize on bolts and studs like this, and loc tite on others.
Ray