Stripped Holes in Cylinder Head for Intake Manifold Bolts

Once there is a hole, a larger drill with proper grind will have to follow that hole. There is nothing to push it one direction or the other. It could be done with a handheld drill and be very close to the same center. I would consider larger bolts (even a metric size) before Heli coils and locktite. Of course you would need to enlarge the holes in intake a bit also.
The highest temp locktitie available might fail in a head, especially near the crossover passage. They get mighty dang hot.