Hensley Motorsports has been building 4" plus bore 318s for over thirty years. I've spoken with them on the phone about it before. While they say they sonic test them all, what I gathered was that most all they sonic tested were plenty thick enough and plenty evenly poured. Even still, they said they'd not skip the sonic test. But they said the "reject" pile was far smaller than the "accept" pile, for whatever that's worth.