Is my 340 Overcarbed? Avg 5-6 MPG

It's ALL in the tune up. It will need more than a jet change is my guess.

Get the initial cranked up and start from there, which bushing on the distributor? Black? You'll need a different one to get it right at idle. I sell them and they work well. If it was tuned on a engine dyno, the in car requirements can and sometimes are VERY different.

I wouldn't worry about changing a bunch of parts, I have stuff with a much larger camshaft that runs well on the street. It's sorting it out that will take some time.