I will be honest, I have only ever owned a NA 3800, Toronado Trofeo.. But have lots of time with the 3.1 Turbo LH0. Did lots of research on the L67 for a Fiero build I was gonna do, but after I got injured, working on "dream" cars became a thing of luxury I no longer had. The W-Body guys have had a lot of time with the L67.
Their biggest downfall was the cars they were working on, W-bodys, they were a LOT of the -fat- side, Fiero although not a skinny gal, is still a lot lighter then those. After that it was keeping axles, and as you mentioned, the 4T65 from imploding. Most have found the comfort zone around 3.2 or 3.0, depending on supporting mods. Some ventured over to a 282/284 manual trans, but they ended up being weaker than the 4T65, and parts are 10x worse to find, plus the custom axles needed.
Last I knew someone was working on a Custom 6T80 standalone kit, but the package would require mods to frame rails, never heard what ever happened on that.
I started doing research on trying out a Aisen TF-80SC, but gave up after my injury. Nice compact 6 speed tranny, and there is a aftermarket for it to make them nearly bulletproof. They were used in A LOT of cars, GM, FoMoCo, Volvo, Fiat, etc. They can be built up to reliably handle 500hp easily, and with certain rebuild parts, 700hp. It does have one downfall thou.. One axle would need to be hugely long, and the other short. As you know this creates torque management issues for the FWD crowd. On a Fiero Application... It raises a bunch of interesting ideas. For those that didnt know about the TF-80SC, their next argument was that it was a AWD tranny.. Which is true.. Except the AWD was a externally mounted PTO, so its no longer an issue. Take the PTO off, find a matching axle spline..