I tried using block filler to stop water jackets leaks on a customers old Mopar cast iron top fuel block that had interior coolant leaks from grinding for clearances around the mains from a different owner.
I ended up having to use a liquid block sealer from Goodson that had to be pour into the block and then sealed up under pressure with a torque plate and water pump block off plates for 20 minutes and then baked in a oven at around 350 F for 20+ minutes after draining the sealer out of both sides one side at a time. It was reusable and it did a good job on fixing those coolant leaks. I did both sides first and then had the block cooked by a local powder coating shop, we pressure checked it at 30 Lbs. for 20 minutes before putting it back together.
The customer put that motor in a car and it is still running today as far as I know