The block doesn't determine the compression ratio. The pistons and heads do. All 340s had the same casting number. The casting date will tell you a lot more about the engine. It's cast right on the side of the block.
front of the block below the drivers side head, should be some numbers/letters stamped in it.
or
pass side just above the pan rail, there is a machined boss with the last numbers of the vin. Get those.
Get those and it will tell what the engine came from or when it was built.