Boostin is correct in the sense that the car is held together by a few welds, spot welds. When they construct the car the Spot weld it which is a process that involves overlaping two sheets or pieces of metal and then passing a localized current through them and fusing them to gether in one spot - normally about the size of an m&m. if you look closely at you car you'll see the small m&m dents where the welds are. They are quite strong and are far apart to let the car flex and give a bit but were never intended for high hp and or track/rally conditions. So what people do is run a bead of weld along the seem where the two pannels meet, usually and inch of weld then two inch gap the an inch of weld etc. This stops the pannels and chassis from flexing and pulling apart but can make pannel damage a nightmare to fix as the pannels are ridged and cant be pulled apart. It would be smart to do it if you are going the turbo ca18 way but as for nessacery? you will have to check with the local laws and requirements.