That motor has Vortec heads and a really small cam. Because of the small cam, changing heads won't net you much. Additionally, the Vortec heads are really pretty good and fully capable of a lot more hp.
So, for better power you'd be looking at a cam change, a few upgrades to the heads to handle the new cam, and programming to the ECU to make the upgrades run correctly.
Do you have someone to do the programming on your engine when you do the mods or will you be programming it yourself ?