Surprise, the deluxe fresh air square heater core for these five years (of the same design cab) are slightly different. They begin by using water outlet and inlet pipes the same ½ inch diameter as most all early years.
Apparently in 1958, these connectors increased in diameter to 5/8” diameter. It is only assumed this allowed more water flow and thus more warm air in colder climates. Viewing later factory heaters they continue with this larger 5/8” diameter on GM vehicles.
Currently, this square truck heater core is under production. A “step-up” rubber hose will be included. Thus, the new cores will be correct for either diameter of water outlet.