Fuel economy is about efficiency, one side is the engine and drive train and the other is the fuel itself.
"Built to stock specs" doesn't really impact the fuel economy side; back in the day they didn't get great mileage either but gas was cheap and much better than what we have today. I would argue that "stock specs" is probably worse for mileage than a more efficient performance build.
Poor mileage is a direct result of the carb, the ignition and the exhaust as well as the quality of the fuel. Using all stock parts is going to make getting good mileage difficult.
I agree that getting an O2 sensor and tuning the engine with that is a step in the right direction. If you want better mileage you need to improve the ignition system and perhaps the exhaust system. You can also try changing the heat range on the plugs, tweaking the timing a little as well.
You also need to rethink those 3.54 gears if you really want "decent" mileage; might think about something like 3.23's. Lastly, you need to ensure your tires are fully (or even over) inflated to reduce the surface contact patch a little.
Good mileage and classic muscle cars are not synonymous typically although it can happen. If this is an issue for you and you really want to drive the car a lot, think about going to EFI and a good ignition like MSD.