Also, this is the same problem that kevjon runs into when hes faced with not being able to do his model with "NURBS".

The curves in your file when viewed with anything other than top view look like this



which in no way have anything to do with a bumper. So you'll be left with making that part up. The original ferrari plans had the curves there in their entirety, but took a long time to get them there, and therefore could create the surface you see on the car.

Reverse engineering a car from curves like this would be next to impossible, but you could get a good approxamation with Sub-D, it just wouldnt be true.