I have a 4T60 in my 86GT. The photo below is of the two types of speed sensors that came with the 4T60. The one on the left is the one you will need in order to be compatible with the Fiero. Most 4T60 transmissions came with the speed sensor on the right.
Good luck. Once you get the 4T60 installed and running you will really like it. Lower engine RPM at highway speeds and the gas milage.