A girdle is going to restrain the motor from contorting. That's going to lower friction considerably in high hp situations and save rods, pistons, crankshaft, and block. I've heard of people gaining hp even from a set of main and valley girdles.
I think the stock block can make it out to upwards of 600. It's all in the girdles and the tune. I'm going to see how far mine will go next year when I can afford to upgrade if I blow it up.