from what I understand, a posi-track differential wouldnt be much benifit to these cars on the street because of the way the engine is situated. with the big V8 engines pointed front to back, you get the torque taking pressure off of one side of the car. with the fiero, the engine shifts back and forth..
however, if you want somthing for "offroad use" (not sure about the legalities of it) you could always weld the differential shut so it cant turn.. (crazy automotive teacher tought us this!).. just an idea.. appearentally our teacher used to do this with the cars he built for drag racing.
but good luck if you plan to ever run that on pavement with a solid rear!
just some ideas on the topic..
hope you find a solution..
jude