I'd vote for the head too.

Just went through a similar experience with a #14 Head SDL. Drove the vehicle for years showing a very steady temperature gauge value. No sign of overheating. Suddenly the gauge starting doing the rapid rise/drop action and heat in the cabin was intermittent. Got it back home and parked it.

When I went to start it the next day the engine was hydro locked. Coolant was low in the overflow tank. When I finally was able to start it, the SDL was billowing steam and there were bubbles in the overflow tank.

Postmortem on the head showed several cracks.