In the parts listings for the Thunderbird, the springs are listed in no particular order, by the spring code, which comes off your driver's door info label, or the tag on the spring.
Ford does not list the springs by stiffness or weight, or even break down the model or engine / trans combos. So looking at the list, the parts person cannot tell you which spring might be for a lightly optioned base 3.8 NA car, which is for an SC, etc. A car like T-Bird, which came in many versions, can have as many as 10 - 12 different front springs listed, depending on how the car was equipped. Ford will even use different left and right front springs on some vehicles.
Just FYI, many of the choices for T-Bird springs from Ford have been discontinued already, and the ones that are left are very expensive. One of the shops we deal with phoned a few days ago for rear springs on a 96 Bird. The ones listed by the spring code (which I got from the VIN he supplied), were no longer available. Out of the 8 or so choices listed for the rear of a 96, only one number was still available. They were about $140 CAN for each spring.
My suggestion would be to get a good used pair of springs from the most lightly optioned base 3.8 Bird or Cougar you can find, or try a parts store for an aftermarket replacment set of springs for a 3.8 NA Bird.
cheers,
Ed N.