THM250 - THM350 - THM400

These transmissions are all very similar to each other externally and in function. They very in strength quite a bit though.
THM stands for “turbo hydra-matic”. People commonly refer to these as “turbo 350” or “turbo 400” for the 350 and 400 models.

The THM350 and THM400 are ubiquitous with drag racing. These are generally considered to be very strong transmissions. For most use cases they don't need to be modified to be able to withstand high amounts of power.

The powerglide was a predecessor to these transmissions and would be adapted in the same way

These transmissions are pretty simple because they do not have any electronic controls. Some may have an electronic torque converter solenoid that you might want to wire up if you want to lock the converter.

Adapting a 3800 to any of these transmissions requires an adapter plate and a pilot conversion hub. Here is one example of a kit with both of these parts made specifically for this conversion
https://www.speedwaymotors.com/Chevy-4-and-V6-to-TH350-Adapter-Plates,3280.html

You may also need to have your flywheel drilled to accommodate the bolt locations for the torque converter for any of these transmissions.

Another option would be to have a custom converter made with the correct flex plate bolt hole locations for a 3800 flex plate.
I haven't looked into this in too much detail yet but there might be other options on the pirate4x4 3800 Bible page