TC springs == Mustang springs ???

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.
 
Well I would guess the back end would be up about 2" higher cause of the longer TC spring, and the front would be 1" because of spring rate.

Disclaimer; this is only a guesstamate. I would not put money on it.
 
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.