Aftermarket cast valve covers can be a problem. All of the ones I've seen have only a bent piece of sheet metal for an oil baffle for the PCV openings. In comparison, steel valve covers have a real baffle that's sort of a labyrinth affair that is designed to separate the oil from air. My new Blueprint engine came with chrome steel valve covers that has the real baffles. I'd like to replace them with something nicer, but I'd rather have an engine that doesn't burn oil.My vote is either a PCV siphon problem or intake manifold gaskets.
Vanity has its price.
Also, we would all like to think our intake gaskets are sealing properly, but assuming that all those surfaces between the heads, and intake are perfectly aligned and fitted is a mistake. It should always be checked before final assembly. Previous machining of the head and/or cylinder deck will affect the fit of the intake manifold. Sometimes the intake manifold will need to be machined to make the runners line up properly.
I had a 302 once with only 60K on the rebuild that started fouling plugs, using oil, and running poopy. I assumed it was just worn out, but it had good cylinder compression. It turned out to be leaky intake manifold gaskets.