I'm not sure, but perhaps you have the belt adjustment in the wrong spot? Check that first...that the notches are where they say they're suppose to be.
Becuase these cars are made of FRP the tolerances might not be great and why there's build variation from one person to the next.
As long as the belt is not touching. Go with it! If it's touching perhaps lightly sand the chassis with 600 to round the edges in that spot.