They are different spring rates for sure, especially the 87-88 birds.
The rear would be up in the air because of length, the front would sit higher because of spring rate.
From what I have been able to gather from CoolCats is that the Tbird springs are a little bit longer. Mustang springs cause the Tbird to sit a little lower than a Stang. I have read that SN95 springs are a direct fit for a Tbird.
I think it would depend what year thunderbird you took them off of, but it would definitely lift the car. I heard that '87-88 Thunderbird springs will lift the car 2". I know that I put a pair of '84 Thunderbird rear springs on my mustang (I wanted to raise it) and it raised the back and least an inch, maybe more.