I weighed the model and it comes to 4.3oz (170g), so it is still 1.6oz too light. I added that via the load by attaching some weights to the bottom of the load. Another solution could be to glue some more lead shot in the bottom of the car's interior, thereby keeping the center of gravity low, since the load will be "permanent" in this car.
This is a list of things I need to improve on for the next model. I have made so many obvious mistakes on this first model that I am actually kind of embarrassed to have this article published here. But, we all have to start somewhere!