I recently built and swapped in a 5.9 Magnum. I also had leaks around the valve covers. I did two things that fixed the problem. 1). I ground all the valve cover bolts to make them shorter. I think they may have been bottoming out.
2). Ran a hose from the breather on the driver's side valve cover to the air filter assembly. Before that, I had a PCV to the carburetor from the passenger side, and just a breather on the driver's side. I found a breather with a nipple to run the hose to the air filter assembly. No more oil leaks.
If you zoom in you'll see the hose leaving the breather.
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