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That's the casting number. I don't believe you're going to find the part number stamped on the block anywhere. What are you looking to do with the part number?
If you tell me the year and what motor it is I can probably get it at work tomorrow, but they should be able to track your part invoices with your truck info wherever the work was done.
They probably don't use the same part numbers as Ford. You'll probably have to go into a Napa and ask to find out. It'll probably be difficult to get a receipt since you didn't buy the motor.