Is my 340 Overcarbed? Avg 5-6 MPG

OK guys, sorry for the long delay in updates, but I was on det to San Diego. Here is the latest and I am making progress:

1) Jetting on the carb was returned to the original settings
2) The bushing in the MSD distributor was removed and a 10 degree was installed.
3) I set the initial timing to 24 degrees and the motor is very happy right now. I verified total at 3000 rpms and it reads 34 with no detonation or pinging. I tried advancing all the way to 34 degrees initial and the distributor never kicked back as stated. It was happy there as well with no detonation or pinging. Right now I backed it down to 24 degrees initial until I get further guidance.
4) Took the car on a test drive and throttle response was very snappy, power was there and there was no stumble at low speeds at all. I took readings from the AFR Wideband during my drive at highway speeds. Here are the results:
- Idle (800 rpms): 14.5 to 14.7
- 1500: 12.5 to 12.8
- 2000: 12.2 to 12.5
- 2500: 12.0 to 12.3
- 3000: 11.8 to 11.9
- 4000: 11.9 to 12.3
5) I am thinking that I need to jet down based on the rich AFR readings, but I am not going to pull that trigger till I hear from you all. My only concern now is that if you do tell me to jet down, I am going to get that low speed stumble back. Thanks for all the assistance.

Ken