Re: BOLT head in the Bilge...
THANKS EVERYONE... Sure got a ton of replies and advice, and I appreciate all of it! Here's what I did...
Lined up the local boat yard to do the work if what I was going to try failed. Then, walked to Home Depot. Bought a bottle jack, a breaker arm (long socket wrench), PB Blaster Penetrating, and a big Visegrip (I only had the medium size). Last night I PB Blasted the bolts. This morning I put the jack in the bilge under the stern end of the engine, jacked it tight, but not enough to move it. Then with my fingers crossed I put the big wrench on the remaining upper bolt and slowly cranked. Woohoo, it moved. I removed it. That was probably a 5 inch bolt with threads only at the end (odd since the broken head had threads right to the head). Next I removed the bracket by taking off the upper nuts on the mount. Then I got the big Visegrips and put it on the end of the now revealed broken bolt. It moved so easily I thought the wrench had come off. I removed it with my fingers. It was only about 1.5" long and all threads. So the two bolts were different. I thought I would be visiting a hardware store or the boat yard for a new bolt, but a quick look in my stock and I saw I had three SS bolts that were identical to the broken one. I used a lock washer and one of those bolts to reassemble.
I suspect that the bad bolt was very tight, and then when the opposite corner was raised during the alignment it stressed the neck of that bolt and it eventually gave way. I'm just glad I found it.