12v exhaust manifold bolts seized and broke off flush with the head

Hi I am new to the forum and I'm trying to help out a buddy "highwayman" with his build. We got the head off of his 12v which came from New York and 4 exhaust manifold bolts broke off flush at the head. We have tried everything from heating them up and welding nuts on them and we broke 3 extractor bits last night. Are there any other specialty tools or advice that anyone has tried or had success with?