I had thought about doing something like this using a 273 or a 318 with a ported SP2P manifold and a small hydraulic roller with the small chamber "302" heads and fariy high compression. I was going to use a OD 833 with the goal of well over 30mpg highway.
I had a 94 Firehark with an LT1 and a 6 speed and that sucker would run 13's all night long and get 31 mpg on the way home. the .29 drag coefficient and the motor turning under 1500 RPM at 60mph made a huge difference. part throttle torque at low rpm and aerodymanics really make a difference. EX the 1960's VW bug had the same chassis and engine as the Karmen Ghia but the KG got something like 30% better highway MPG.
