Getrag F40
Overview
The Getrag F40 transmission is a transverse manual 6-speed transmission. It is cast aluminum and certified for up to 295 lb-ft of torque.
Strength
There are many who consider this transmission 'unbreakable', however there are also many who believe it has not been tested as thoroughly as other options either.
Some have reported reach 600 ft-lbs of torque at the wheel with this transmission though.
How to make it work
Use this information at your own risk, this may be incomplete or lacking in some areas
The F40 will bolt up to the bellhousing of the 3800, however you will need a custom clutch to send power through it. Spec is one company that makes a conversion clutch though for it.
You will also need the correct shift cables for it, possibly custom length axles and your own transmission mounts. It's pretty far from a bolt-in swap in most 3800 cars.
The F40 is quite large due to its 3-axis design. Because of this you may have issues with clearance around the frame rails.