Here's a great site for Ford part numbers:
http://www.mustangcentral.net/tech/part.html
If it's a 351W from a Fairlane, the number on the block would be:
D2OE-6015-XXXX
If it's a 351W from a Galaxy, the number would be:
D2AE-6015-XXXX
The second four numbers, 6015, indicate 351W.
Stroke it to 408 for around 500hp
Pete