This gets talked about regularly on this list and the Unofficial Allis board too. The cracks do not hurt anything - IF - they are at the top of the block. The head gasket takes care of sealing it all up. If the cracks are at the bottom of the block where the o-rings seal the bottoms of the sleeves, the block "might" be able to be welded.
Check the Unofficial Allis board at the link below for more info. Harrell Sellers is the expert on this over there.