1965 Barracuda with 84 318

Well, there have been some big cracks in engine parts before! Possible a freeze up situation. I have never looked to see how close that passage is to the cooling jacket in the head casting

And these engines sitting for a long time can get internal corrosion. Within the last year, someone here showed a cylinder that had coolant weeping in through a pin hole from the cooling jacket that had developed.

I guess I'd suspect & pressure test the head 1st. (Maybe because it is easy to take to the shop...)