I've found that the cast in bolt bosses in the lifter galleries are sometimes protruding past the gasket surface of the block. That will make the cover bottom before the gasket is making a good seal. A straight edge run along the gasket surface will show any that stick out. A few minutes with a file will trim down the high bosses. Pack the area with rags to keep the filings out of the engine. Flush to about 1/16 inch lower than the gasket surface is good.
Also check to see the cast alum side cover is flat and not curved or warped. If the holes in the side covers don't have a chamfer on the outside similar to the dimple on the factory steel covers then you can make one with the tip of a 7/16 or 1/2 inch drill bit. This will cause the rubber washer to be worked into it and seal snugly around the shank of the bolt rather than have a tendency to squeeze out from under the head of the bolt. Chamfer the hole to about 1/3 of its' depth.
