What got me thinking was.... when people swap 4.9 in Fieros, they can keep them either auto, using the 3speed Fiero tranny, or 4speed using the Cad tranny... OR make it a 4/5 speed manual using either the getrag (sp) or Isuzu tranny.... I would think then you could get something rigged up in a Cad to accept it also....
I did a bit of research on this for my Allante. Here's what I've found:
There are really only two GM 5-speed manual transaxles that will bolt up (with modification for the starter) to the Cadillac block. They are the Getrag/Muncie 282 (the 5-speed used on V6 Fieros and Berettas) and the Isuzu used on I4 Fieros (and other vehicles, I suspect).
Other potential transaxles are the Muncie 4-speed used on V6 Fieros, and the ultra-rare Getrag 284 used on the 5-speed GTP and Z34 with the 3.1 circa 1989-1992.
Couple of issues:
First, the "easy ones".
1) You would have to modify or otherwise adapt the speed sensor
2) You would need to fool the ECM into thinking the transaxle was attached (for when it attempts to electronically shift it)
3) You would have to adapt the gear selector and neutral safety switches
1)Two of the three transaxle/engine mounts would have to be completely fabricated.
2) Driveshafts would have to be adapted/fabricated as the distances would all be different
3) The manual transaxle doesn't have a "snout" running across the back of the engine. Berettas had either a pillow block or a cast extension.
4) A flywheel would have to be custom made (or gotten from the Fiero guys)
5) The transaxles have to be cut to accept the stock Cadillac starter.
Even if you could accomplish all that...
1) Fieros have a hard time getting the clutch to handle the torque. There's not much space in there for a larger diameter one. This is a problem because
2) Cadillacs weigh a lot more than Fieros. It takes more torque to accelerate them, all things being equal, because there's more weight to haul around, thus more torque has to be transmitted and the clutch is already borderline, plus
3) Its easy to spin the tires in a Fiero with a Cadillac engine. This acts as a "safety valve", preventing the full torque of the engine from being channeled through the transaxle. That's not going to happen on a FWD Cadillac and it increases the odds of breaking something.
So, yes, you could probably do it. It may even last a long time if you don't abuse it, and rebuild the trans before installation. There are a lot of issues to sort out though. I suppose the first person that does it will gain some interesting experience!
Another avenue I've thought of pursuing was to adapt the manual trans off a Taurus SHO. That car weighs about the same as the Allante and it might last a bit longer than the Getrag 282, and certainly the Isuzu. Then, though, you have to deal with physically bolting it to the engine, and mixing Ford and GM driveshaft pieces.
Part of me would like to give it a try when I finish remodelling the house. If the trans wasn't such a PITA to drop out of the Allante, I might think about it more.