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.

~~ Jethro
1954 3100
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In the Dity Gallery
1951 3100 (gone) / 1956 4400 (still in the neighborhood) / 1957 6400 with dump body (retired) / 1959 3100 panel (in the woods junked) / 1978 Custom Deluxe K10 / 1993 S-10 4.3 / 2004 Chevy Crew Cab / 1945 John Deere H / 1952 John Deere B / 1966 John Deere 2510 / 1967 John Deere 1020