Hi and welcome to Inliners. The mexican blocks are the most desirable, as they tend to be a little thicker than those cast in the US. As for the block cracking issue you have described, I have only been aware of the shorter deck engine blocks like the 230's and 250's cracking into the deck around the water pump area if they are decked too much. That seems to be in the same area you mentioned, so maybe thats what you heard was actually meaning. But I haven't heard of that being a problem with the 292 blocks.
