Franz at Big Toys in Australia, has just created an adapter that allows one to connect a GM 700R4 four-speed automatic transmission to a 6.3 (MB M100.981) motor.
He is also working on similar adapters for the 6.9 (M100.985) and M116/M117 (380, 420, 560, etc.) motors.
You can see all of this at:
http://www.big-toys.net/cgi-bin/detail.cgi?0&49
Cheers,
Gerry