Large rear weight box bolts to the lower 2 bolts that hold the axle housings on and the front two lower reduction housing bolts. This box is precision laser cut out of heavy 7 gauge steel and welded together. Large box size measures 10-1/2" wide x 17" long and is about 5" deep. Box has a removable rear cover to load weights in an out of at the track. This box is a fully welded piece. If you would like to save some money and weld your own together the parts only are available in the drop down options box.