My thoughts and experience, exactly. I had to put my car out for an entire winter (before it was finished) and the cover I had, which was mainly a dust cover and a blue tarp kept it dry. You can even strap it down under the car if it has grommets.
I agree with the silicone and metal sandwich gaskets, but the ones that came with the cast aluminum valve covers from Mopar Performance were great too. They were kinda thick black ones.
You should vent the crankcase and eliminate the possibility of any pressure. Also, most valve covers will have a baffle (or plate across the opening) where the holes are in the valve covers. Since yours are cast, there may be a place to screw a plate. A small plate made of sheet metal will suffice.