Chris I am no certainly expert, but most Cobras don't need massively high spring rates, particularly if there is a sway bar. The lower weight of the car makes that possible. For example a typical 'street' Cobra might have 300-350lbs at the front and 250-300 at the rear (remember that Falcons supposedly have spring rates less than 200lbs). For racing these might increase, depending on the shocks/sway bars used. Some are using 400-500lbs. If most of your driving is on the road then you don't want it to ride like a board. There was also an earlier thread that may help:
Shocks and Spring Rates