https://www.stovebolt.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=616193#Post616193http://www.chevsofthe40s.com/shop/s...p;dept=Truck&searchtype=FT&Go=Go (they have one style that is a reproduction for the original accessory - it's no less expensive than the one at the link above).
I have never had to use an overflow tank with a 216/235/261 engine. You fill the radiator to within an inch of the top and run the engine for 30 minutes after it has warmed up. Let it cool down and then check the level. A little fluid might overflow until the correct level is reached. When cool, the coolant level is about 2-3 inches below the filler opening.
Until 1954, the radiator cap on a truck was 0 pressure; later, they were 4 psi to 7 psi, depending on the year and size/duty of the truck. A high-pressure radiator cap on these trucks would most likely cause leaks at a few weak points in the system. Shops/people who are used to modern, high-pressure, cooling systems almost always recommend overflow tanks for older vehicles.
