A pair of tension adjustment blocks can include one or more positioning members receivable in the tension adjustment block mounting aperture to define a selected one of a plurality of distances from the aperture to a corresponding end of the tension adjustment block. When the tension adjustment blocks are selectively arranged in one of a plurality of orientations with one of the positioning members inserted into the tension adjustment block mounting aperture of each of the side rails to define a selected distance, the corresponding end of each tension adjustment block retains a header at a selected fore/aft position related to the selected distance that is different from the other adjustment tension adjustment block orientations and related distances.