40 projecting above the upper face 54 as shown in fig4 of the drawings the ends 56 of each support rail 42 are seated in the mounting channel 45 to the first frame members 14 in a close fitting relationship as shown in fig2 and 5 of the drawings the first frame member 16 which forms the front frame member is seated in the mounting channel 56 of the first frame members 14 in a like manner to that in which the opposite ends of the support rails 42 are mounted the frame member 16 is however also rigidly secured to the frame member 16 by means of a mounting bolt assembly which is generally identified by the reference numeral 58 as shown in fig6 the mounting bolt assembly 58 includes a tubular sleeve 60 which has a threaded pin 62 projecting radially outwardly therefrom a bolt 64 is threaded through the sleeve 60 and through passages formed in the end wall 65 of the ramp 20 and through the side wall 21 of the front frame member 16 the head of the bolt 64 bears against the wall 65 and a nut 63 is connected to the other end of the bolt 64 and bears against the side wall 21 this serves to clamp the ramp 20 to the front frame member 16 and it also serves to position the sleeve 60 so that the pin 62 fig5 extends through a passage 67 in the wall 69 of the frame member 14 a nut 71.