You don't have to rebuild it. The current download is already built with 6.4.

The problem with floating is due to the way the wheel size is calculated. I am enclosing the entity in a box and then measure the diagonal in 6.40. In earlier versions I instead used the longest side of that box. There's no difference in 2D, but in 3D, the wider the wheels the more the car will float.
With the old method, however, non-spherical entities would instead penetrate the floor. I am not sure yet how to resolve this.