I too have a '85 GT and am about 2 weeks ahead of you in answering this question.
I ordered the Big Fiero Parts Book. On page ACE-6, it lists transmissions that will fit a Fiero. The 5-speed Getrag 284 is rated as the highest reliability. It was available in 1991-1993 3.4 TDC V6 motor in a Cutlass, Grand Prix, and Lumina. Expect to pay BIG BUCKS (approx. $2,000 or more). The 5-Speed Getrag 282 is rated as high reliability and is much more common. It was available in 1989-1993 3.1 V6 motors in lots of things - Beretta, Cavaliers, Fierenza, Sunbirds, Pontiac 6000's, etc.
I found a Getrag 282 from a 1989 Beretta and am working with Rodney Dickman to convert. So far, he has been very knowledge and helpful. He has a conversion kit, and list of additional parts needed to make it work.
I plan on installing the tranny over the next few week-ends. Let me know if you would like progress reports.
Hope this helps.