The motor has to be put together right or they leak. If you have to much
oil in the heads you can not keep it in.. Miller Machine in Haverhill MA did mine. Phill is an old drag racer and has forgotten more about FE motors than I will ever know.
I use permatex (red) to stick the gaskets to the Valve Covers, don't over tighten the the valve covers (the alumium covers are truer than the steel)
I run Diesel Motor
Oil which I think is a component of the no leak FE.