[Dead Parrot]

My response to your original post was based in part on your expressed need to avoid long delays in finishing your house. Broken bolts are part of the old vehicle experience. Some are easy to fix, others not so much. Depending on room available between your motor and radiator and your flexibility, getting that bolt out and fixed might require removing the radiator and other things on the front of the truck, which could take a lot of time away from the house. And once the bolt was fixed, no guarantee that your motor woes are over. Could easily turn into a week or more project and if your vehicle repair luck holds, could be longer.

Things you might try on this bolt if you decide to attempt a fix.
Strong magnet might pull the remains out if it is broken.
Could attempt to weld something on the head to pull it out. But if it won't pull out, you now have something else in the way.
Crows foot wrench would hold the bolt head in place while you drilled a hole for an extractor.

If you get all of the remains out, as others have suggested, you can retap the hole. The threads were tapped when the crank was made, they can be retapped.

As for what caused it, while lots of folks, myself included, use that bolt as a handy way to turn a motor over, don't think that is a factory recommended use. It is really there to make sure the damper stays in place. Also possible a previous owner installed a standard grade bolt.