[Tinker]

I've always, when the heads are off, taken a long screw driver and/or coat hanger and dug up casting sand out of the water jackets. Amazing how much can be left if the block has never been boiled. Just probe around with the coathanger and flush it out.

You might have an exhaust leak also heating up the coolant. There are a few ways to check and maybe someone else can explain it better then I will.

One way to check is to remove the top radiator hoses with water in the block and look for bubbles.