2004 Monte Carlo SS supercharged with 145k miles: I just replaced the upper intake manifold and supercharger gaskets...plus the oil pan and transmission oil pan and dipstick gaskets. I'm still getting white smoke coming from the rear of the engine.
White smoke almost always means you have a coolant leak into your combustion chamber. If it wasn't happening before working on the intake manifold gaskets, I'd suggest you created a path for coolant to flow into it somehow. Not sure on the 3.8L SC engine how the coolant runs, but this would seem the most likely cause of your white smoke. You'll most likely have to pull your work back apart and see if you can find where the issue resides.
If the white smoke was occurring prior to your endeavor and this was the reason you were changing the gaskets, you may have other issues, like a blown head gasket.