Yeah I think a 4.500 stroke crank in a 0.030 over stock block makes 557. Aftermarket blocks can go bigger but I'm not sure how easy it is to find aftermarket blocks these days. Ford used to make the A-460 block but I think they stopped making it. We could've built a 557 but the 521 seemed easier. Stroker kits and pistons were available and the CR was in line with what we were trying to do.