The thickness of the web casting is thicker on the earlier blocks. Any 50557(will have to check this number) casting is the thinner webbed casting. There may be others. I have seen this personally and can confirm. I have also read that there may be a difference in the iron alloy used. This I cannot confirm. That being said, you'd be fine with the later block on a standard hp rebuild. Butler advised that anything under 600hp is OK.