Not exactly an answer, but may help each other
I have read about Fiero seats in a GT. The guy said something about wanting the head rest speakers. I've also heard of stock Miata seats (with head rest speakers) that were successfully installed in a CONVERTABLE GT, so I pulled out one of my Mazdaspeed racing seats to take a look. My mounting brackets are off about 2" front and rear, but I can make new ones since all of the adjustability is in the seat itself. The height of the seat scares me more than anything. Have you taken out one of your seats to have a look yet? There is very little room under the stock GT buckets and I'm not sure yet if I'll be able to get the new brackets low enough to the floor to keep head room and still clear the seat pan to slide for and aft. Is the Fiero seat dished down or fairly flat? I'll attach some pics so you can see everything I'm discribing, and so you can see why I want to get this particular seat to work. The bolstering is exceptional and trimmed in leather with a cloth center section. If they go in, I will probably dye the center sections black to match. If you have the Fiero seats already, I would suggest pulling the passenger side on the GT and take a look at how things line up. You shoud be able to make the brackets work but an aftermarket set up may be easier if you don't have the time or tooling to work with. With my project I want to keep the car side of things as it is, but if this is of no concern to you it may be easier to modify the the floor pan underneith to get the desired mount locations right. I'll keep you up to date on how things go with mine if you're interested. Hope this helps. Feel free to ask questions if you run into any problems or need advise.