Unfortunately someone used an incorrect bolt to fasten the heads to the block.
Take a blunt object a solid bar the same or larger size than the head of the fastener. Give it a few whacks with a BFH.
Try attaching a vice grip as hard as you can to the head. Have one person rotate the vice grip while the other rotates the allen tool. It helps to try to rotate first tightening a bit then counter clockwise.
Hope this helps
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