So recently purchased a new car, 69 charger R/T original motor although known to have been rebuilt although unknown what was exactly done. (Was running a Holley 850 with 80/80 jets and the previous owner stated that the gentleman he purchased the car from had the engine balance and blue printed and thought it may be stroked.)

When I drove the car home,gas spraying the car following and whenever engine turned off it would diesel and exhaust smelt strongly of gas.
Assumption was engine heavily over carb'd. Checked plugs to confirm and they were saturated.

Purchase a holley street avenger 670, looked to be about right size if engine internals are stock to mild as I did not want to go to aggressive as its the matching numbers engine for the car. (Engine has miloden pan, headers, performance intake, stock heads, 1.5:1 rockers. About all I know without taking engine apart.)

Currently I have the new carb jetted with 68's in the front and 72's in the back.
Air fuel ratio at idle is approx 13.25, but even adjusting air mixture screw on carb I cannot get it to fluctuate much from this point.

What could cause this problem, jet sizing? Faulty exhaust air fuel guage? Should I go up jet sizes? (70 fronts, 74 rear?)
Thought?

Too much fuelpressure?
13.2 AFR is pig-rich.

I'd think it'd be lean w/ 68-72 jets. I ran more in a mild 340 w/ a 750Hp carb. I'd look elsewhere for your issue. You probably want to start at 15* initial and 36* total all in full at 2200-2400rpm, then set your mixture screws for best vaccum. The holley 850 would be fine, you'll just have to tune it. Probably something like 72-76, 31 squirter.

running very lean will cause the o2 sensor readings to read richer because of the incomplete burn of the air/fuel mix. you might be running very lean.

Steve if you cant figure it out on your own the guy who just finished working on my roadrunner's carb is always an option. He is a whiz at this stuff and made a world of difference on my car's drivability. It's amazing how well it runs now.

""13.2 AFR is pig-rich""

To determine what is rich or lean for each setup depends on a few items:
- gasoline, E10, E85/compression/octane
- no vacuum advance, or vacuum advance for cruise
- altitude/heat/humidity

All of these items can affect what each combo can run and because of the above, there is no single A/F number that is going to be rich or lean and work for everyone. If you change one or two of the above items, then the same combo may require a different A/F value.
For example, someone running gasoline and no vacuum advance should be able to run leaner than someone running E10 with vacuum advance.

seat-or-the-pants is the only A/F needed. Tune it and drive it hard. Keep tweaking until it falls off and then go back. It's not rocket science. A basic 440 should blow the tires up w/ about 10 minutes of tuning and nothing else but a 1/2" wrench, flat head screwdriver and timing light. If you want all out performance, take it to the track with a box of jets and have fun. Don't over think it.

Stephen, You're really asking two questions in one.
The most efficient AFR varies with load placed on the engine. The exact number will vary as others have pointed out. Generally as load increases, AFR should go leaner. Then somewhere around 80% load, it needs to get richer, much richer, so by max load its somewhere between 12:1 and 13:1.
Max load for a car will be 3rd or 4th gear, Wide Open Throttle.
On an engine with a hot cam, idle usually needs to be richer than a stock cammed engine.
13.2 will not be fouling the plugs (carbon covered), but less than 12 generally will.

The second part of your question has to deal with understanding the different circuits of the fuel curve. My advice is to get a copy of Urich & Fisher's Holley Carburators and Manifolds. No BS, no wondering if the advise is good or not. Urich was VP of Engineering at Holley, and the diagrams and explanations are top notch for a basic understanding of how any fixed venturi carb works.
You can find a g**gle book version, or go buy the real thing, used or new. Older editions just as good as newer.

Then you'll be able to use some of the tuning advice that others have posted in this thread.