Having just bought a set of new front springs, I searched the forum for info on proper size. Found several different things, in the forum, including this:
"MAC's told me that the company who makes the coil springs is Coil Spring Specialty and I just talked with them. They said their coil springs for the Squarebirds, are constructed to OEM specs at 15 1/2" 8 1/2 coils. They also said that originally, Ford was putting 14 1/8" springs on the Squarebird, but then changed to the 15 1/2" length later in the Squarebird production runs. He also said that the information in the Ford parts book might have been correct at the time that book was published. He said many of the early Squarebirds had 14 1/8" front springs installed in them, but the front coil springs they supply to Squarebird parts houses are the later Squarebird production run uncompressed 15 1/2" springs."
I bought my springs from Larrys TBird which got them from Thunderbird Headquarters. The new springs match the revised numbers as indicated above.
I'll also attach a chart that was posted in the forum.
If anyone has recently installed these replacement springs, it would be it would be interesting to know how the ride height ended up.