My 340 getting 6 mpg

There is nothing wrong with your carb other than FST pays low IQ morons to put them together.

I can tell you without looking at it that:

It has too much emulsion.

The idle feed restriction is up high when it should be down low.

The main air bleed is too big.

It needs T slot restricters.

You’ll need a timing curve with 24 initial and a curve that is slow through peak torque. Vacuum advance would be a bonus.

I see all the Carter boot lickers have been here. That’s the LAST thing you’d want.

I had a 350 Chevy on the dyno Tuesday (a 1406 Edeljunk turd) and it was so lean neither one of us had the jets or metering rods to even get it to make a pull.

Learn to tune and fix what you have. Ignore the knaves who tell you that a Holley won’t get fuel mileage.