Just Purchased Virgin 340 Short - What to do with Pistons

Block number is 2780930-340-7
That's the casting number.
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On the other side of the Block, will be the casting date and time.

Engine number, including VIN or partial VIN was stamped onto a pads.
Look on the front, below the heads.
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and above the pan rail on the right.
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Some examples with decoding comments here:
Mopar Lost and Found