I'm not sure what tranny is behind that 7.3(there are 4 different auto trannies for that truck) but I would just use the ford trans. The older fords used a C6 behind the diesels and it worked ok.
That should be a good swap...I have an old '73 K20 that I have thought about mounting a 4 door cab on and shortening the box to fit with the 4 or 6 cylinder Cummins diesel. The 6.9 or 7.3 ford would be just as good(less mpg though).