These holes are drilled through the side of the block and then on through the siamese portion where each cylinder is common. Maybe start and inch or so above the oil pan rail and move upward in 1/2" to 3/4" inch increments until you are maybe an inch or so from the deck. Then drill and tap the outside holes in the block to plug them. The holes that go between the cylinders are maybe .060" to .090" in diameter. Whatever you can find that are that long.