Their pieces replace the front and rear quarter panels and both bumpers. They bolt on in the stock locations, but you would want to bond them to the frame with and adhesive as well.
Obviously, if it is wider than the stock body, it is going to add some weight. However, you can get 18 x 12 wheels with 335s under the rear fenders. It also looks like the car should have from the factory. It is not something you really notice until it is next to a stock car, then all of a sudden the old body looks really wussy.