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knapikmichael,
Welcome to FTE and the IDI diesel forum.
The C6 is the strongest automatic that was used behind the diesel.
You should be able to find one in a salvage yard rather cheaply.
The C6 was vacuum controlled so you will also be doing some vacuum line work.
When you call the salvage yard, you need a C6 that was originally behind a diesel engine, 6.9 or 7.3.
I am not much on automatics, but there are several other people here that can answer more of your questions.