I would ask what they mean by a stock block. If they mean stock fasteners than yes I wouldn't go any higher than 500hp, but then I wouldn't go any higher than stock hp with stock fasteners.
I have studs, both head and main and have my block half filled with hard blok so technically it isn't "stock" but then again it isn't a high dollar race block and it didn't take much work for it to handle the 550 I am putting thru it for 3 seasons.
Like 340srule is saying, it is all in the details.
Chuck