I'm pretty sure Ford still casted the actual numbers under the starter next to the oil pan rail well into the 90's. Check there. And IIRC it should say "351" in the lifter valley with the intake off if it's a 351W. But they could have deleted that casting later on. Easiest way is to measure the deck width and compare it to a 302. I have a couple torn down 302s to compare to if need be.