Kelheor,
So the issue actually isn’t directly related to vehicles, but with welding in general.
The reason your having issues placing the box in different spots is that the mass of the collision volume (even though it’s not simulated) is still being added into the vehicle, and offsetting the center of gravity calculations. Therefore, placing the cube over just the front or the back will cause extra forces on suspension.
Unfortunately, this is a known limitation of the engine right now. I’ve entered a ticket for this.
Thanks,
Jon N.