Its usually on the drivers side below the injection pump. The numbers are cast into the block and are about 1 inch tall, they usually stick out about 1/8th to a 1/4 inch. If you don't see any number there, odds are that the block was cast in Mexico. However the 53 has been known to be located else where, but I've never personally seen that, I think its pretty rare. And I believe the other place it appears is on the passengers side below where the heat exchanger would be. Below the turbo and rearward on the block. However I'm not 100% sure about that. Also, just because it is a 53 block doesn't mean that it will crack, even if its been highly modified and pushing a lot of fuel.
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