Well if the block is thick,and you have a factory forged crank plus good rods hell push the thing and see what it'll take! I know the factory 440 forged cranks routinely withstood 700 h.p,not sure what a small block crank will take.
I've heard several builders say 575-600 is where they would push a SB but it's all relative (parts, machining, etc). If we start talking R-series blocks and serious internals then the 700 range is doable.