I had a motor mount bolt shear off in an engine once. Like Julian said, an oxy-acet torch will do the job. I did not have one so I took it to a shop who did. They heated up the area and then cranked up the O2 causing the bolt to pour out of the hole. The block was and threads were left nicely intact. Sounds like this is just one option of many for you. Good luck and make sure to post your results.
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