69 383 Water Pump Housing Casting Defect?

Sorry for the long pause, have been very busy this last week. Last night I drained the radiator and filled the system from the heater core hose until it reached the heater hose outlet on the pump housing then plugged it and continued filling until the coolant came up through the radiator cores. Finally I put the core hose back on the housing and sealed everything up. I wasn’t very hopeful since it seemed that the coolant level was the same, but we took the car for a 20 minute drive last night and the temp stayed under control, going 205 F at most. It would drop to around 195 when cruising. This morning I noticed the coolant level had dropped quite a bit and put about a third of a gallon into the radiator. Took the car for a 15-20 drive and the temp never went above 165F…. Which was exciting and concerning. It was 65F outside, and there is a 180F Mr Gasket Hugh flow thermostat in the car now. Hood seal is on order too.