A harvesting machine includes a harvesting header with a header frame, a flexible cutterbar assembly attached to the header frame along the length thereof and configured to cut a crop, a plurality of laterally spaced apart support arms, a conveyor assembly, and a cutterbar drive. The support arms are pivotally coupled to the frame for swinging movement about a laterally extending axis so that the flexible cutterbar assembly is operable to flex along the length thereof in response to changes in terrain as the header is advanced. The cutterbar drive includes a drive element and a forward gearbox drivingly connected between the drive element and the cutterbar assembly. The forward gearbox is supported on an outer support arm.