weigt is best added as spring weight and best in rear of car bumper ,trunk ect.Check with official to see whats legal bolt in or welded,maybe even a weight box with lead shot.
400 lbs is a lot to add. You'll need to spread it around some. In the rear just ahead of the rear axle, some in the middle maybe behind the seat or on the passenger floor, and some in the front....
Whatever you add as weight to the car make sure it's bolted or welded to the floor or chassis. In case of a accident, that stuff flying around can be lethal.