There have been two ls6 valley covers. The early one used an external PCV valve and the later ones don't because they have a built in orifice. If you bought yours new recently you probably have the later style. To tell, blow into the hose connected to the tube on the front of the cover. If you feel a restriction to flow, you have the new one and no valve is needed.
I did this swap on my '01 Z28 a few years back with the new style cover. It didn't take care of the oil in the intake problem for me. Recently bought a Mighty Mouse catch can and will see if that stops it.