I agree with everybody else that your gas milage appears a little low, and there is probably a little room for improvement. But here is the deal. It is very difficult to build an engine for performance AND at the same time get good gas milage. Here are some thoughts:
1. You said it was a 'mild' 318. One person's idea of mild is another person's idea of moderate. We know you have headers and a dual pumper carb. We don't know what you have for heads, intake, cam, or compression ratio. So the less 'mild' your engine truly is, the more power it has, and the more fuel it will use.
2. That is an unusual carb for a 'mild' 318. A 650 double pumper is definitely more performance oriented. how is it jetted? If you jetted the primaries down, you could cruise using less fuel, but still have power available at WOT. Every time you step down on the gas pedal past a certain point (maybe just to pass or zip through a yellow light) the accelerator pumps in the secondaries may be squirting in extra gas. A jetted down 650 vacuum secondary carb would actually give better fuel milage and still have plenty of power when you put your foot in it. You might even consider a smaller 4 barrel considering how mild your engine really is, and what your performance/milage goals are.
3. Although 3.23s are higher than 3.55s or 3.91s, they are still considered to be a performance gear ratio. I think it is a very good ratio, but 2.76s would be better for milage.
4. Lastly, we do not know how you drive. A heavy foot will yield far less fuel milage than a lighter foot.
I can only imagine how expensive gas must be in Australia. But overall, if you drive a modified classic car with a V8 and a performance gear ratio, I don't think you should be worrying about gas milage too much. I have a 69 Barracuda with a 340 that is around 375HP, a 4 speed and 3.55 gears. I don't measure my gas milage because I don't care. I measure Smiles/gallon, Thumbs Up/gallon and Waves/gallon. Don't get me wrong; I am NOT belittling you for wanting better gas milage, I just think that if you are going to drive a classic car with a modified V8 engine and a performance gear ratio, you should not be worrying about fuel milage too much.