You must have a PCV system hooked up otherwise you run the risk of blowing out gaskets and it will always leak oil no matter how many breathers you have, imo..

Here's how I did it 25 years ago with cast valve covers. Drilled the right sized hole for the grommet with a hole saw. And bolted in this baffle. I used this valve cover for 20+ years on my car. The baffle worked great.

I learned about PVC valves on my first 340 which came with these cast covers with no PCV hole. They were black and shinny like yours. I fought oil leaks for a while until a mechanic told me it needed it.
So drilled that hole and got a grommet out of a stock valve cover and it stopped my oil leaks and more. Made a difference for the good and I became good friends with that mechanic. You can get that grommet at most "help" sections at auto parts stores. All PCV grommets were the same in those years.