Don't use valve cover gaskets !
You will have endless leaks on the top of your engine making a huge mess
The factory used RTV/silicon.
Clean all surfaces and make sure the valve cover is flat especially around the bolt holes.
Use an appropriate size bead on the surface,set the cover on and tighten the bolts a little,let the RTV set up overnight and then finish snugging up the valve cover bolts in the morning,or appropriate time for the RTV to set up.
90 Chev 3500 c/c 4x4,6.2na,400 auto,4:10 gears.DSG Timing gears,main girdle, isspro tach, pyro,boost,oil and trany temp.Dual Tstats, High volume peninsular pump,on shelf, Custom turbo and intercooler 85%complete. Change of plans for the dually, it's going to get a Cummins. Both trucks are Blue 90 4x4 crews