69 383 Water Pump Housing Casting Defect?

I'm assuming you have flushed and flushed and flushed the block and rad? an easy way to be sure MOST air is out of the system is to fill it on a slope nose up and fill through a radiator hose about a foot higher than the block. then have the other rad hose about the same level. When coolant comes out of the other hose you SHOULD be full. and coolant should be coming out the rad cap opening too