Maybe dyno it and see what the torque is?
I was mucking around with a mate (legally) yesterday: He has a 351 Cleveland backed by a T5 + 3.23 diff, I have a 302 Windsor with the A4OD + 3.27 diff. I have a bit more power than him, and he has a lot more torque than me (both engines are mildly modded). Needless to say he slowly pulled away from me
What I'm getting at is, if you get it dynoed and it has a torque reading that's much higher than what a standardish/mild 302 should put out, that should give you an indication that it's a 351W, or at least a stroked 302 (347?).
But do what Lukeh suggested for the best result: find a mate with a 289-302 and compare the dimensions. It's free, it's quick, and it's decisive.