If the front sway bar does not provide any locating function for the front supension, try disconnecting it.

I am not familiar with the Contour rear suspension so I will ask you--where are the springs located (inboard or outboard). Your rates might not be that far off if those springs are outboard (e.g., an outboard coil over) so that they are near equal to the effective rate. If inboard, you probably could use some more rear spring.