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Tremec T56
Overview
The T-56 is a legendary transmission in the 3800 world. This transmission is known for being nearly bullet-proof in stock form and offers plenty of aftermarket support. There are few who have done this swap though still.
The T-56 is a 6 speed manual longitudinal transmission. It was the standard transmission behind a lot of LS-based V8 engines. You can find this in Camaro's, CTS's, Corvette's, and many other RWD performance cars from GM.
Because of its demand it is hard to find one without investing a significant amount of money. However compared to the cost to build a 4t65e, this cost is pretty insignificant.
Strength
The T-56 is rated to handle at least 450 ft-lbs of torque in the 98-02 f-body's. There are countless examples of people who have pushed this transmission well beyond this range.
Usually most issue come with higher mileage and abuse. There are many who have pushed over 1000 horsepower through a T56.
How to make it work
Use this information at your own risk, this may be incomplete or lacking in some areas
You will need to use an adapter plate to make this transmission fit your car. In the US you will need to look at the aftermarket for such an adapter plate. In Australia it seems like there is an OEM bellhousing that will bolt up to both the 3800 and the t56.
The aftermarket adapter plates would either be installed on the bellhousing or the mid-plate of the transmission depending on what route you choose to go down. If you use a mid-plate adapter you allegedly could use the f-body t5/3800 bellhousing and T5 clutch for an option that nearly bolts together.
Again, you will need to do your homework though here. This has not been documented very well yet.
Malwood appears to make an adapter plate that might work: https://malwoodauto.com.au/product/adapter-plate-kit-for-holden-v6-to-suit-ls1-t56/
Other sources I still need to comb through:
https://forums.justcommodores.com.au/threads/vt-v6-6-speed.229878/