Is my 340 Overcarbed? Avg 5-6 MPG

I think I'm starting to agree with moper here unfortunately it looks like you have some parts mismatches. That carb is very big unless you're turning 7000 RPM and I think the overall calibration of the carb to your smaller engine is too far off to be worth the trouble of tuning it IMO. I bet an OOTB 670 would run better than your current setup with all the changes.