GM spec (for cork) is:
Valve Cover Bolts 65 in.lb = about 5-6 ft/lbs.
Most important is to make sure you install the spreaders correctly. If you have a set of OEM spreaders, look at them closely. You'll notice that they are not flat - they are cupped.
Install them so that they are concave down (convex up). The construction of the valve covers - thin sheet metal and minimal fasteners- makes it hard to keep a tight seal throughout the operating temperature range. The spreaders are cupped to help maintain a constant spring load on the valve cover flange.
If you haven't already done so, go out and buy a set of PermaDryPlus gaskets. I have used them exclusively for a few years and never get even the slightest leak. Insructions say to install them dry, no sealant and no adhesive. But for ease of removal I use a thin bead of silicone on the valve cover side. They have compression limiters at each bolt hole. Makes tightening them foolproof - using only your fingers just reef down as hard as you can. Go back around and retighten after a couple of weeks. Use 1/4" stainless steel lock washers under the t-bolt faces to help maintain torque.
