Chevy 350 electric water pump extra hole on the passenger front side of the block where the water pump mounts there is another non threaded hole under the bottom water pump bolt hole.?

I have had a problem with my engine overheating and then suddenly cooling off when first started up after the thermost had closed or was closed, and found that the stop leak I had used to seal up my leaky heater core until I replaced it was the problem. I was informed that it had gotten in the water pump at what I was told was the pressure bleed off (The extra water hole on the passenger side of the pump/block and was causing enough pressure to hold the thermostat closed until enough pressure on the other side of the thermostat therefore relieving the pressure holding it closed. I went through everything, and found the problem fixed after cleaning out the hole with compressed air ?? crazy but it worked. PS. New engine, new radiator, 3 new thermostats new heater core, left only the water pump wich was new but reused for rebuild. I did however not have the cooling problem until I used the leak sealer on the old engine with new water pump then the problem started. I will never use that stuff again !!!