My family had several Darts with /6's, and out of the auto trans cars, my mom's 1980 Aspen with the 2 BBL Super Six got better gas mileage than her 1969 Dart 1 BBL. I understand that the 1980 Aspen had the lean burn system, and it certainly had a locking torque converter, and I am 99% certain that it had a lower rear axle ratio, so those are unknown factors in the equation. The Aspen was also significantly 'perkier' than the 1 BBL, even though it was a bit heavier.
Results are going to depend on a lot of factors. Just the rear axle ratio, and the resulting RPM's on the freeway will make a big difference, as that sets the cruise vacuum level and if the power valve in the carb is open while cruising. And, how you drive it and how you tune it up makes a big difference. So without knowing those factors for a given situation, like for Slantsix64's car, you can't draw broad conclusions from one setup.
