Is my 340 Overcarbed? Avg 5-6 MPG

I think you are mis understanding what is being said about advancing you INITIAL timing.

You are advancing you Initial timing by turning your distributor! You need to replace the bushing in you distributor to get more initial timing.

Example 18 Initial the bushing in the distributor advanced timing up to 36* AKA you bushing is allowing 18* advance.

If you change the 18 degree bushing with 14* bushing, now you set you Idle or Initial timing to 22 degrees and your total timing is still at 36'

I know you are resisting taking the distributor apart to replace the advance bushing but it MUST be you next step.

Trust me your seat of the pants will tell you you did the right thing.

You can't decide what PV you need, YOU can't decide what jet you need UNTIL YOU CHANGE THE ADVANCE LIMITING BUSHING IN THE DISTRIBUTOR.