Not sure from your description if the engine was run with the radiator in place since the core support is not installed. How was the truck stored after the owner passed on? If it was outside for very long the water may have come from weather or condensation over time.
I would drain the oil, install the radiator and hoses, fill with water and do a pressure check on the cooling system. If the pressure falls, check the oil pan for water. If any is present then I would suggest a rebuild or swap out.