I'm not sure the most common crack comes from porting, but more from the abuse of racing. Big block heads in general are known to bust right below the two center exhaust ports. I think by the time these heads reach 40 years old, have been milled, slapped on a engine with high compression AND a full exhaust system their chance of making it to 50 years old is not good. Not to mention the compression is trying to lift the heads off the block and the combustion chambers themselves are oil canning. Its a hard life and they bust plain and simple. Let alone all the places that get thinner from the porting.