The leak required to pressurize the cooling system is minimal...
The area on that fire ring is mighty close to the coolant passage.....

The condition of that fire ring is pretty sketchy.....

Sure looks like a good possibility to me...

Getting the heads and decks cleaned up well with the "Sanding block" should give you a good idea of what you are dealing with.....

I would do a good wipe down of the cylinders and look for anything hinky ....as in strange stains or discoloration ......

Just to be sure and cover the bases...