3.5L runs a different plug, and has different CAN protocols. The 2.5dd, 3.0dd and 2.5det all run the same ecu plug with slightly different pinouts.
Most 3.5L ecu's will work, without cam actuation. If you want the cams to work, you will need to reprogram the frequency/degree and self learning, which only the Haltech Elite can do afaik, and without Haltech support you are up the creek (as Mercury found out the hard way). Short story- Don't bother at this time.