Years ago, maybe 15+, Car Craft took a 63-64 Nova and pulled the straight 6 and put in a very respectable 350. They dramatically increased their fuel mileage. They say it's because they needed less throttle input to move the car, which equates to less fuel. I believe it was a 194 and a powerglide that they replaced with a 350/350 combo. You gain more torqu going with more inches which helps get you off the line without having to step on the pedal so much.