4T80e

The 4t80e is the strongest 4 speed automatic transverse transmission option available. The 4t80e was common behind Northstar engines in front wheel drive cars so it has the same bellhousing as the 3800 and should be able to fit in most 3800 platforms.

The 4t80e weighs 245 lbs with the converter compared to the 4t65e which weighs 195 lbs with the converter. Some have claimed that the 4t80e takes significantly more power to drive than the 4t65e as well.

Though mechanically it will bolt up, the electronics are an area where a lot of people start to lose interest in the swap.

LS4 V8 block mated to a 4T80E four-speed automatic

The 4t80e was rated for 305 ft-lbs of torque, though some have estimated that it may be strong enough to handle 600 ft-lbs in stock form.

It is far stronger than a 4t65e even in stock form and can be upgraded for much cheaper than a 4t65e.

This image shows the difference between the two in size:

Use this information at your own risk, this may be incomplete or lacking in some areas

The rear knock sensor on the 3800 will need to be removed to make room for the transmission
The rear lower bellhousing bolt will need to be grinded a little bit as well.
You will need to make a bracket to support the tailhousing

The rear exhaust manifold seems to interfere with the transmission as well. You may lose some clearance for the downpipe since the transmission is so big.

The flywheel will need to be drilled since the torque converter uses 4 bolts instead of 3. Spacers may also be a good idea.

Axles:

  • Will need an Aurora 4.0 axle on the driver's side
  • Will need a stock deville passenger side axle

The stock 3800 pcm is capable of controlling the 4t80e just fine with some tuning work, however you may need to do some homework here.
You could also use your own electronic switches to trigger the shift solenoids manually for manually controlled shifts. You will need to make sure the shift pressure is set properly though manually since the PCM won't be controlling it in that situation.