A chassis frame (10) uses channels as right and left side rails (12, 14). A lower “clam” (22) has a horizontal bottom wall (26L) that bridges the side rails and is disposed against bottom flanges (20R, 20L) of the side rail channels and vertical side flanges (28L, 30L) that extend from opposite sides of the bottom wall upward and against a face of a respective one of vertical walls (16R, 16L) of the channels that is exterior to the interior of the respective channel. An upper “clam” (24) is a mirror image of the lower “clam” and fits over the side rails. Spacer structure (30, 31, 38A, 38B, 40A, 40B) is disposed between and in abutment with the side rail channels and between and in abutment with the “clams” in inwardly spaced relation to the side rail channels. Fasteners (48, 50, 56, 62) fasten various parts together to complete the cross member that is formed by the “clams” and the spacer structure.