The stock auto is on the threshold at anything over 400 rwhp from what I have seen.
You need to look into
http://www.specialtyz.com They offer a fully built auto trans with a billet flex plate and precision industries torque convertor that will handle about anything that you can throw at it. They have it in their 9 second street Z. We have installed several of them, the shifts are incredibly crisp and the stall brings boost on really quick.
Z1 motorsports also offers a built auto trans, I don't have any experience with them though. I haven't seen any feedback about their trans online either.
Tim