Just add the com value to the returned object in init_chassis:
return {mass:1500, com:{z:0.3}, steering:2.0, steering_ecf:60, centering: 1.6, centering_ecf:20};
The value of com says how much to shift the center of mass from the model's pivot in model space, so z:0.3 moves it up by 0.3m from the pivot. Setting com:{z:0.2, y:0.5} would also shift it 0.5m towards the front side.