I found this technical drawing of a Dart
http://www.autotribute.com/wp-conten...t-Drawings.png
There's not a lot of aft-body for the car and plan taper starts rather late,leading to a relatively large wake.
Thule makes a 1-inch receiver hitch type bicycle rack system which could be modified to create a slip on bustle/boat tail extension for elongating the car.
Something like this would help recover pressure,raise the base pressure,cutting pressure drag which Hucho says is the entire premise of streamlining.
Front skirts,rear skirts,and tail should show significant savings.An inspection of the car's underside might reveal some wiggle-room for belly pan sections.
AeroCivic would be an outstanding benchmark to model after.