Are these stock type stamp steel valve covers?
If so make sure the gasket surface the V.C. gasket seals against on your covers is flat and straight, no dents, low spots or high spots scope
Once you know more let us know what you find, them DANG HEMIROID motors can be a pain sometimes whistling grin
AKA aside, you can fix it up wrench

Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)