Ford calls the heat riser a "Shroud and Tube Assy" - C4OF-9A607-A, and a "Duct and Valve Assy" - C3AF-9A626-B. They are supposed to assist the engine rise to a good operating temperature along with the choke by ducting warm air into the carburetor. When the engine reaches it's optimum operating temperature, the choke is open from heat entering via the fitting on the RH exhaust manifold (289 engine), the flap valve closing off the warm air from the same fitting. In warm states, they operate quickly. In cold areas, it may take forever.
The exhaust version was not used (thankfully) in a 289.
Jim