Ridergary, you were absolutely right. I looked closer - a LOT closer - and the gasket was inside the head. If you don't know what you're looking for, you'll never see it - I was expecting gasket material, not metal. And the valve cover doesn't have a gasket apparently. I flattened it with some 320-grit sandpaper on a piece of glass and it isn't leaking anymore.

Things are looking a lot better. I think my major problem was the valve timing and backlash. I also had a bent pushrod and a too-small prop that incidentally had a pretty good nick in it that I didn't think about - I've since been reading about aerodynamic balance on props and I think that might have had a lot to do with the vibration. I put on the choke assembly and a new 14x6 prop and she runs MUCH better. Thanks to all of you for helping out - I think this one is going to have a happy ending.

Thanks again.