Diesel Dodge Dart Concept

The 4BT is only about 200 lbs lighter than the 6BT, not a lot of gain there.

I'd have a long look at the Merc OM-617 turbo diesels. 3.0L, and out of the box performance about equal to a /6. Lean on them a little and I think one will run away and hide from a slightly leaned-on /6, and get better mileage doing it.