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Are these just visible cracks or have you had the block magnafluxed and/or pressure tested? If these tests haven't been done, it would be foolhardy to proceed with any machine work. I doubt that any machinist that doesn't know how to bore a block to fit existing sleeves has any idea about cracking problems with flathead blocks, much less how to fix them.
I agree with the rest of the guys, find a new shop.