with the 882 heads you are pretty much limited. I think you would be lucky if you were making 300hp. you have to remember that the heads you are using where used on 350 motors that where making 160hp. to double the hp that those heads were originally intended on making would seriously be doing something. I can't find the exact numbers but i dont think those heads where flowing more than around 180cfm @ .500" lift.
if you were willing to spend around 800.00 for a set of aftermarket vortec heads like the RHS pro torkers, I think your hp would go from around 280-300 up to around 350-360hp