Heater core Is def trash, we did the heater core cutout back in high school. I’m just going to assume that the radiator is trash, and add it to the list of stuff to be upgraded. After taking the water pump off, the block looks like it’s full of sand, or dried hard water type stuff.Woohoo!!!! Another hurdle jumped!
As said above that's why they are called freeze plugs. Just make sure you check the heater core and radiator as well. If water was in the block it was in them as well. They don't have freeze plugs though.