Anybody break a stock block, if so at what hp?

I use the 600hp level as the stopping point when I'm working with somon plannig big power. If you're planning for more, budget for a better block. Stock blocks flex, and more importantly the cylinder walls flex. You give up power before they break, normally. A lot has to do with the casting quality and machining quality. Great machining can enable an average block to perform better long term. But eventually everything will give up. Poor maching can break anything...lol.
All that being said - I was involved with one that makes 556 NA at the crank, and another 200 on the plate. In a heavy, all-steel street car with a cage it went low 10s on spray.