Yeah, canadianal, those old crystalized bolts twist off easy, don't they? It's a common problem with the big Cadillacs. They run a little hot anyway, and If somebody ever tried to lean one out more than normal to try to get better mileage the exhaust gas temp gets really high.

I twisted off five or six bolts before I broke down and took my 500 heads to the machine shop. I learned a long time ago that you get some very unkind comments frrom the machinist if you take him a bunch of botched attempts at drilling out broken bolts or studs.

Good luck.