Need recommendation for a Carb for better MPG!

A Street Demon 625 would be a great choice, but 17 is pretty good for a 340. What gear do you have? I think expecting 22 or more from a 340 is something out of a fantasy movie. That 1406 is already an economy carburetor. The 1405 is the performance version. I'd check your tuneup first. Where is your initial and total timing?