A harvesting header includes a header frame, a flexible cutterbar assembly attached to the header frame, a draper assembly supported behind the cutterbar assembly for conveying severed crop materials received from the cutterbar assembly, and a plurality of laterally spaced apart support arms attached to and cooperatively supporting the flexible cutterbar assembly, with each support arm being pivotal about a lateral axis. The header further includes stop structure corresponding with each of the support arms, with the stop structure located forwardly of the lateral axis and defining a limited range of angular support arm movement.