the basic part number digits for the 302 are 6010 and 6015 for the 351W. If the motor ever gets pulled and you have a chance to look the motor over these numbers are going to be down where the starter is. The starter will have to be out of the way to see the numbers clearly. Example: My 351W block is a 1969 casting and the part number on it is C9OE-6015.

 

I've got a 351 CLEVELAND, all my engine info is on the driver side valve cover, I'm not sure if it helps, thats just where mine is on my cleveland. I didn't know they put them in 250's, I just thought they put them in torinos and rancheros.
My 2 cents