You are looking for a problem that doesn't exist. A good waterpump flowing through a good radiator with clear engine passages doesn't care if the truck has a heater or not. Most 261s were used in big trucks running full loads against a governor allowing 3500/4000 rpm's all day long. Many didn't even have a heater, which was a dealer-installed option throughout the 235/261 production years. They came from the factory with pipe plugs where the heater hoses go.
