Or tune the engine correctly. Probably has an off-the-shelf "performance" carb that's set much richer than it should and a "full race" cam that only makes sense for a track car, giving horrible fuel economy and actually less low-end power.

My daily driver is roughly as old, has a 400 V8 with a 4-barrel, idles so quietly I've had passengers surprised that the engine was running, and gets around 20-25mpg if I resist the urge to open it up all the way.