I used to have a 1975 Buick Skyhawk with a similar stepped back due to louvers on the back window. I think you're better off modifying a CRX hatch, myself.
Yeah your right a CRX would be a ton better. So for know I'm going to finish this car and see what it will do at 45 mph. Then try the step back at 65mph and see if it will get over 70mpg. If not I will re-group and figure out what to do at that point.