My 340 getting 6 mpg

MPG is tough. My right foot dictates. 16 MPG is the best I can get on very modest highway driving. Stockish 440 4-speed B-body running Dana 3.54:1 with talker 28" rear tire. But just a couple fun thumps on the pedal and suddenly drops to high single digits. Or worse if nonsense continues. Lol.

My 440 is running single pumper 750 Holley. I found for best performance and economy? The smallest carb that still gives you the needed CFMs at WOT. Faster throttle response with better fuel economy.

I would think the 340 as described? A 650 spread bore may do that job?