Probably best method is birail, but you´d need to revise all curves to intersect first working on a duplicate. After this you can also stitch some parts of the model.
Anyway, real cars are made of independent pieces, and they do not need to match exactly with a perfect parametrization. Most importat is curvature and control over the model.

Also try to align the surfaces.

It looks good. Is it an eco-car running without petrol?