There are several reasons for the Continentals to have cracking problems. These engines were designed and produced in the post WWII era wen the Allies bombed most of the world"s industrial capacity to dust. There was great pent up demand for manufactured goods in the post war years also. The combination meant that factories were running overtime to produce machinery for domestic use and to support the Marshal Plan for a recovering Europe. Most likely quality control suffered and the casting technoloy of the time was not nearly as good as it is today. Add to that a design that did not take into account this impact of the casting technology and the QC that didn"t look very hard for these things and it"s no wonder there were a lot of marginal blocks produced and shipped. The result is that these Continental engines are prone to cracking.The likely failure modes were just out and out weak webs between the cylinders in the early failures and fatigue failures in the same section of the older blocks. The original Z129 on our TO-30 had the tension rod fix on it and it worked well with the fix but was just plum wore out when I replaced it wth a remanufactured block that had a furnace braze repair on it.
The N series blocks were based on the Model T engine and they were well below the state of the art in engine technology even in the "40"s. But they were a proven, robust, thirty year old design that could be made in vast numbers. They were ~ 20% poorer in output at the same displacement than the Continental Z120 mainly due to the superior volumetric efficieny of the Z120"s overhead valve design.
Those are some of my opinions regarding your question.