Valve Cover Gasket Sealing Issue

2 things are at work:
1) Sheet metal valve covers are usually warped/bent. Try to straighten them out.
2) Most stock heads are left "as-cast" on the valve cover rails. Some are a lot worse than others. Not much you can do.
No guarantee these will "fix" the problem, but I always use 93050 Moroso Perm-Align gaskets now.