That type of block cracking seems to be very prevalent in 216 blocks. It happened to my 216 also in the same relative location as yours. I sawed my block through mid-crack just to see if anything particular about this design stood out. To this backyard mechanic, I couldn't see anything unusal. I suppose the original casting process or metallurgy can account for this happening so much, but I don't know for a fact. And of course age and use can damage the block.

Drew