Cast Aluminum Valve Covers Leaking

Had a grungy set of M/T valve covers on the engine and replaced them with the new polished Mopar ones I got with the car. The old ones never leaked, they are just ugly. Used new cork Fel-Pro gaskets and ARP bolt kit. First, it took several tightening cycles to the bolts to hold torque as the gaskets seated. Now the bolts stay tight, but I've still got a leak at the rear of the head. The leak runs down the block and drips on the headers and it's getting anoying seeing and smelling the smoke. Haven't pulled them back off yet, but I looked at the old covers and saw that one had a black rubber gasket and the other had two cork ones siliconed together. Obviously they had issues in the past. What is the formula for success here? I don't want to glue the cover onto the head but some silicone between the gasket and cover should help?