On that year block, check and double check your tester. If made in China don't trust it. I had a 400 block tested at the machine shop and it showed the thinnest spot to be just under .250. Had it bored .040 over. Assembled a killer short block. But ran across a killer deal on a KB low deck block. So I sold the short block. It never made it out of the dyno room, several of the cyl's were paper thin. Still feel bad about that deal. Moral of the story, don't trust the sonic checker. If possible find someone with a known good tester and recheck. If it sounds too good to be true. It usually is not true.