I ran into a problem like this on my 428. It ended up being I was using valve covers for non adjustable rocker arms with adjustable rocker arms, and the adjusters were rubbing on the baffle inside the valve covers. Fortunately, I caught this during the first 10 seconds or so of breaking in the engine, and there didn't appear to be any metal shavings or anything in the
oil. A swap to some taller valve covers cured my problem. Don't know if this is what's happening with your engine, but I hope it helps.