Chingadero;
Ford came out with the 351-W in 1969,the 351-C was made for two or three years around 1970 thru 1973 I believe...
On the 351-W, if my memory is correct,the casting #'s are on a small "pad" near the
oil filter boss.... They usually have two sets of #'s. The one starting with a letter (C9xxxx) is the one you are looking for... C is for the 1960's,the number next to it is the year of manufacter (9,1969)... Starting with a D is the 1970's and so forth and so on. A D 2 would be 1972 and so forth and so on.
Not real sure on the Cleveland blocks were the numbers are,but Ford used this type of code for most of the parts,,,blocks-heads-intakes....
Hope this helps....
David