A method of manufacturing a box header for an air-cooled heat exchanger. The method includes the steps of boring at least one hole into at least one end of an elongated slab of metal. Material from the slab is milled out adjacent to the at least one hole to form an elongated internal chamber in the slab having at least one open end. An inlet nozzle opening is machined into the elongated chamber and an outlet nozzle opening is machined into the elongated chamber. A plurality of tube holes and a plurality of plug holes are drilled and tapped in the elongated slab into the internal chamber. An end block is welded to the at least one open end in order to form a closed internal chamber.