[WSSix]

x2 or since you're an auto, a better torque converter and a retune of the shift points. Might as well use whatever power you have now more efficiently.

Don't forget the other stuff that really wakes these engines up like headers and better exhaust if it doesn't already have that. You also have intake and throttle body changes. LS6 intake alone will work well. A small cam wouldn't hurt but I'd change the converter first before I did a small cam.