this may help:

scale to size such that you can use cruller donuts dipped in chocolate for the tires (for the super over size tire, aggressive tread look)

cutout base is raised on four dowels, one each hidden behind a tire. (best to have a full size plywood bottom and screw/glue dowels to it and then screw/glue the shaped board to the tops of the dowels.)

cake itself is four layers -- two for main body (bottom) and two for the cab area. adjust length of top layer to mimic type of truck you need