My 340 getting 6 mpg

Learn how to tune the carb and if you’re not running ported vacuum advance with around 45-48 degrees of cruise timing you should be.
Maybe, just maybe you should watch some David Vizard and Unity Motorsports Garage videos. There are a bunch of others also that speak to ported or manifold vacuum. The short speech; manifold vacuum with an adjustable vacuum advance so you can dial it in.