The engine casting number is behind the starter motor, and the actual stamped engine number is at the back on top of the block behind the 'v', directly forward of the bell housing mounting face.
This site is quite good for identifying ford v8 castings
http://www.cehighperformance.com/for...codes.htm#351W
and this one is a cracker fpr all sort of stuff:
http://www.fordification.com/engineID.htm