Check the rear end, where the control arm bolts to the body. If the car was abused, that 'box' will bend & left the control arm sag a bit. If it isn't straight, you can get reinforcement plates to weld to the factory bracket, car a battle box. Also, the passenger side is the side that takes the stress during a drag race, with the torque of the engine forcing it down. To cure this, some drag racers put a small air bag inside the rear passenger side coil spring, to toughen it up without changing the ride height. That could get yours riding level.