I tried to buy a twin turbo 6.9 with a torqflite about a decade ago bt could not get an agreement to the seller. I guess it's doable. The kid that does some side work for me mated a 722.6 his 300TD and also did it on a W126 350SDL. The transmission will require a stand alone controller which is available. An adaptor plate will also be required to make it work with the earlier style bell housing bolt pattern. The Mercedes Benz 722.6 is a seriously strong transmission.
With best regards
Al