hot air shroud
Ford called these a Shroud (Carburetor Air Cleaner Air Duct), and they cover the exhaust manifold to direct hot air to the air cleaner's inlet so the Air Cleaner Temp Sensor can do its thing to regulate the temperature.
Ford's generic part number for the Shroud (Carburetor Air Cleaner Air Duct) is 9A603, but here are the complete part numbers for Bullnose trucks:
These are pictures of the M-block shrouds, but those of other engines work the same way.
