Its been my experience that most people overtighten the bolts for the steel valve covers, distorting the mounting rail.
I was watching Graveyard Cars one night and ole "cousin Douggie " had a great tip which I now use.
A little skim of gasket adhesive on the gasket and the valve cover , then, set the gasket into place in the valve cover and let it set up.
Then, a skim of adhesive on a properly cleaned cylinder head surface and install the cover with hand tightened bolts being careful not to squat the mounting rail.
Works for me.