Leaks from the rear of the valve covers have driven me crazy for years on my 1965 Newport. The problem appears that the heads don't have a flat sealing surface. It is a rough casting and drops away ~1/8" at the rear. I have negligible blow-by (new short-block & restored heads), so the crankcase even runs at a slight vacuum w/ the PCV. I have tried everything, including the foam silicone sealant, yet it still drips oil onto the starter. Last pass, I tried Real Gasket, which is firm silicone, under a cast-aluminum Holley valve cover. I thought to first try tightening it more at the rear to insure the gap is closed there. As I tightened the forward nuts, the ear on the rear of the cover snapped off, ruining the cover. The surface is that non-flat. Now that I have a milling machine, my "final solution" is to remove the heads and try to mill a flat surface all around. That may put the interface below the oil level, but shouldn't matter. I will also mill off the casting line which tends to stop oil from draining back. I tried that before w/ heads on engine, being careful to collect all shavings (stuffed rags & vacuum). If I botch the heads, I will switch to aluminum heads, which wouldn't be a terrible result (save ~50 lb).