The top or centre oiler has a groove counterbored into the block that carries the
oil behind the #3 cam bearing to the bottom end. It needs 2 holes and no groove. The #2 and #4 cam bearings need three holes with a groove between two of them to feed the
oil to the heads. Just look over the
oil system carefully and follow the oil through the motor to be sure it's getting everywhere it needs to be. There are several theories and modifications you can make to the stock shells and their oil hole sizes. One of the most commonly recommended oil system modifications is to enlarge the 3/8" oilway from the pump to the filter to at least 7/16" and port match the ends. For a street motor that doesn't see sustained revs the stock centre oiler with reasonable bearing clearances and a good pump will provide plenty of oil pressure. I got my cam bearings and anything I can from Jegs.
Cheers