SAE 2 adaptors for the 5.9 are worth big money due to the guys putting 13 speeds in dodge trucks.
The Cummins engine is no SAE pattern. It has a adapter bolted to it. There are many different adapters. Dodge had their own to mate to the Getrag 360 ( 5 spped first gen, and the NV4500 (5 speed second gen) and the NV5600 (6 speed second gen).
Medium duty stuff like buses use SAE 3 and SAE 2. Unlike in the light duty trucks where there is only the adapter and the transmission, the Medium duty trucks use three pieces for more versatility.
You have the engine adapter, the bell housing, and the transmission.
On auto transmissions, the bell housing contains the torque converter.
On standard transmissions, the bell housing contains the clutch and flywheel.
I'm sure I have seen MT643's with SAE 3 torque converter housings.
I have also seen combo's like a DT 466 with a SAE 2, a slim ring to adapt it down to a SAE 3 to connect a AT545 to it.
Industrial application's like generators are a completely new set of adapters.
Here is a chart to help you figure out what your new MT643 is.
Hope this helps.