Definetily a freeze crack, how is the rest of the block? Usually when you see one there are others I had a 426 street wedge block that was crack on both sides, insides and out as well as having a casting flaw on the bottom of one cylinder that had been leaking into the crankcase The customer wanted it fixed so he could use it in his 1965 Satelite 426 W car I would check with your local machine shops and see what they will charge you to stitch plug repair it The shop I use was so much a inch, it is not a welding process. They drill the crack and insert a twist off plug and then drill out half of that plug and insert another twist and so on until they go past the entire length of the crack It flat works well, no leaks and no problems