In my opinion the problem is the newer valve covers on older cast heads. The cast heads (906 etc) have imperfections, and he newer valve covers are so ridged, they do not form to head at all.

I think most Edelbrock head owners will agree they don't have leaky valve covers.

I found that a good quality cork gasket worked better than rubber on a cast head. If you have a standard no maintenance camshaft / valve train, consider using a little RTV.

-dan