My 340 getting 6 mpg

I’m thinking its two things it’s your timing you need more initial 15 initial and 14-16 mechanical

And carb double pumper equals 2 accelerator pumps which drinks gas. Also 67 jets that’s way too small I would think your engine would want 70s

I have a 68 340 10.5 compression also similar cam to yours it takes 16 initial and 14 mechanical I get 12-14 mpg 4 speed 3.55