Pre-Magnum LA engines oiled the shaft-mounted valvetrain via these oil-feed passages in the block deck. The Magnum heads' valvetrain gets oil through the pushrods. If you're starting with a bare block, the oil passages in the block can be tapped and plugged. If you're replacing pre-Magnum heads on a short-block, the factory oil hole will seal against a dead end when meeting the head's deck surface. A light smear of silicone here wouldn't hurt, particularly if extra-thin head gaskets are used.