My 340 getting 6 mpg

Definitely have some tuning to do, you should be getting closer to double that for mileage.

For reference, the 340 in my Duster is a '68 340, .060" over, 9.8:1 compression with 308 heads, 2.02/1.60 valves, 65 cc's chambers, stage II ported and flowing 264 cfm at .500", it has a Lunati 60404 Hydraulic cam, .513/.533", 276/284 advertised duration. I run it with an Edelbrock Air-Gap RPM intake, Holley Ultra 750 double pumper, headers, 2.5" dual exhaust. Timing is ~18° initial, 34° all in mechanical.

Even before when I was running it with no vacuum advance, 3.55 gears, 26" tall rear tires and a 4 speed on the highway in with mixed driving I was getting about 12- 13 mpg.

Now that I'm running it with the vacuum advance hooked up with a T-56 magnum, 4.30's in the back and 27.3" tall tires I'm still doing 12-13 around town and I can do closer to 14-16 out on the freeway if I'm in the overdrive gears.