If the block has the early style 2 bolt motor mounts the casting number will be on the right side of the block behind the generator. There are some later blocks that have no casting number. In mid 68 there were a bunch and also the service replacement blocks from the 70's. The service blocks have vertical ribs on both sides. All blocks have a date code. The early blocks till 64 have them above the oil filter mounting pad and the later below the oil filter adapter pad. Once the engine is removed it will be easier to see. G.