Is my 340 Overcarbed? Avg 5-6 MPG

There are plenty of examples to prove the power valve must be below idle vacuum rating is not so. It's not the issue while cruising on the road.

19/1.5 is more than 5-6... :)

I'd turn up the initial timing to the 22-24 range, get a bushing that limits mechanical to 10* and see how it changes the idle quality and cruise. Find the point where the engine kicks back when hot, that's your limit on initial timing. I would think that set up would pull closer to 10" maybe 11 if initial was raised.