remove exhaust manifold bolts

You have to be referring to the studs at each end of the LA head. end in water jackets. They always break.
Here's what you do... Bolt the exhaust manifold back on the head. Now you have a 3/8 diameter pilot hole for centering your 3/8 drill bit or better yet the center drill with 3/8 shank ( I forget the number on that ).. Drill the casting and broken stud about 1/8 inch deep. Now you have a good center to guide 1/4 inch drill bit. drill through. 5/16 tap to clean up remnants in the threads and you're done. Don't forget the RTV on the new studs.