The nickel content of the block is what is different here,,,,most blocks from the mid sixties and on,,,,they cheaped out on the nickel content,,which makes iron harder,,,,,four bolt mains where the cheap way out for the car company's to sattisfy the power market.
Old two bolt main blocks where Way Stronger,,,than even newer blocks with four bolt mains.
it used to be that we could tell what the motor came out off by the difference,,,four bolt setups where usualy from trucks.