If gas mileage is the goal, an E-body isn't the best choice from an aerodynamic perspective. If you want to stay with an older car, a Duster or an early (1964-66) Barracuda are better choices. Both cars have a fastback style roof and both would have a smaller frontal area than an E-body. Of course if one of the Plymouth Arrows from the 70s' could be found that would probaby be an even better choice. (V8's can be swapped into the Arrows) I'd guess one of these smaller cars may be worth 1-3 mpg.

I'm not a Turbo guy, but have read it's possible to use a Turbo to increase mileage and power. The turbo is susposed to reduce pumping losses, thus increasing fuel mileage. A turbo charged 273 might be a good combination for power and economy. I don't know if a turbo can be sized and set up for economy without EFI.

The thread on the 29 mpg Barracuda shows what can be done using a lean mixture. I've read that turbo charged engines do NOT like lean mixtures. So I don't know what would have happened if a turbo would have been added to Dave's high mileage Barracuda that had a carb on it. Could a carb supply a lean enough mixture for mpg at cruise speed and transtion to a rich mixture quickly enough when the throttle is opened? Or for that matter, is it even possible to run a turbo charged engine with a real lean mixture at cruise speed without detonation problems?