Old Fart here I learnt loooong time aback to knock out all welch plugs and get in and clean out water jacket and always new water pump. = old holdens yes even greys. And old fords. It freaked Me Out when i saw how they blick up Even lower jacket drain plug blocked. Old MECANIIC taught 20 year old ME well. I rrmember that kind old man to this day 😮
To clean the cooling system run about 3-4 red dot dishwasher tablets and run the car up to temp and the slight abrasive of the tablet will basically lightly scour the inside of the block, heads etc, I do it in trucks especially when an oil cooler lets go and then puts oil throughout the cooling system