We had a similar issue, an external hairline crack. Ours was on 216 ci engine installed on our 1948 Chevy truck. We researched the issue and that there is company in California (I thin) that does Marine / Ship engine repairs and they sell a kit for repairing engine blocks. The process was beyond our technical capabilities and we didn’t pursue it. However, we did find a powder sold by a supplier in Europe and we did purchase it and added the product in accordance with their instructions. It took some 10 miles of running and the seep reduced and now so far is not seeping.
