A harvesting header frame has a top beam, a bottom beam, and a diagonal truss member. A first end of said diagonal truss member is structurally connected to the top beam and an opposing second end of said diagonal truss member is structurally connected to the bottom beam. The header frame also has a diagonal truss length adjustment mechanism that adjusts the length of the diagonal truss member to selectively lengthen or shorten the effective length of the diagonal truss member to modify the concavity of the header frame after the top and bottom beams and diagonal truss member have been connected. In one embodiment, the diagonal truss length adjustment mechanism is a telescoping compression member.
