Did the penetrating oil thing but wrung off seven bolts removing the left head bolts using a breaker bar. Bought a battery powered impact wrench for the left head. It has three power settings. Started with the lowest power setting. Each bolt got one second tightening followed by 2-3 seconds loosening. Repeated at least five times. Some came loose on the low power setting. Then went to the middle power setting and repeated the back and forth thing with the impact wrench. All bolts came out without breaking. Pretty sure the bolts would break if I had to go to the high setting. The machine shop extracted the broken bolts, using heli-coil to fix most.