If you're lucky the oil pressure relief valve in the oil pump is stuck in the open position or someone forgot to replace the 9/16 cup plug in the oil passage that separates oil going to the filter from oil leaving it when the engine was rebuilt. You can check the plug by removing the oil pressure sending unit and sticking a coat hanger wire down the hole. The wire should bottom out between 71/2 in.-711/16in. from the deck if the plug was installed correctly. If the plug is missing, you will have to pull the pan, oil pump and rear main to replace it. If its the pump, the rear main stays in place. If its not one of these I think you may have a real problem. Good luck and let us know what you find.