The previous entry about laying out a plan view of the car is directly applicable when you are making a car with a flat base to the chassis, ie. cut from a sheet of plywood. If you are going all out and will be making a “curved in all 3 dimensions” body, its still good to have that plan view layout. It may not be the base of the car but will probably be a plane on which the axles will be mounted. Its important to have a reference plane if you want front and rear axles to be aligned. The soapbox derby construction manual on classic soapbox racer construction says more about this. Its a very good document to read.