Around the city, you can probably expect about mid-teens mpg. Installing a overdrive tranny isn't going to help in the city & a 4bbl carb will get better mpg's than a 2bbl as long as you don't get into it (since you're only using half of the 600 or 750cfm, it's still less then a 2bbl cfm which can be between 375 & 530cfm) Ultimately, if mileage is your biggest concern, I recommend considering a swap to fuel injection, but if this is out of the question, then make the engine as efficient as possible by (1) Keep it in good tune, (2) definately swap to electronic ignition (at least a pertronix), & (3) make sure air can get in & out of the engine as freely as possible (K&N filter & free-flow exhaust). I would install a vacuum gauge in the cockpit to monitor the tuning of the carb. Lastly and very importantly, maintain proper air pressure in your tires, & you can even go a couple psi higher to decrease rolling resistance.
Fuel injection will net you the best mpg (prob 22 mpg city, 28 hwy), and it will eventually pay for itself. If my stang was a daily driver, especially with your kind of mileage, I would certainly have fuel injection.
If you don't take the time to do it right the first time, how are you going to find the time to do it again?