From a voice of experience, a Dakota 5 speed WILL NOT work into an A body.
The tranny is @ 2" longer in the worst place it could be, at the back of the case, right around the tranny mount.
Major restructuring ot the tranny/torsion bar support/floor would be necessary.
My best advice for fuel mileage is to use either the 833-O/D, or use an early 3.08 1st gear gearset from a '64-'66 6 cylinder 4 speed and 2.76 rear end gears, all behind a well-tuned V-8.
Mark.
