Trying to find fuel leak

I love math. Lets see; drove for an hour; that would be about 60 miles at 60 mph. Ok used 1/2 tank; that would be 5 gallons. So 60 miles at 5 gallons. that would be 11mpg. So unless you have a potent big block, or a very potent small block, or race gears in the back,that is excessive. Its so excessive, that even a warmed over 360 would be acting like a pig.So, lets assume, for the moment that the gas is not going through the motor. That just leaves; into the motor or external.Since the OP says no signs of leakage, that just leaves into the motor. Theres only two ways into the motor.Through the carb or through the mechanical fuel pump.Since the OP didnt mention anything about how crappy it runs or idles, Id go with dart67 and the pump diaphram theory. But,here it comes, theres almost 3 gallons missing. If it was in the crankcase, the Op would have noticed the sluggish engine and likely the excessive exhaust smoke.
So,Im going with an external leak. And heres why; the OP did mention new tank and sender.And heres another but;I m guessing thats not relevant.It would be hard for me to imagine 3 gallons leaking out with no signs. Im thinking its leaking on the pressure side of the pump, while the engine is running.Then the airflow at 60 mph is taking it away. The smell when stopped is the last of it evaporating from a place you cant see.
Now theres only 2 places that you cant see; on top of the tank, and just below the fuel pump above the K. Since theres no fuel return on a 69 Dart 273,Above the tank is out. That leaves the area under the pump.
-So, my guess is the pump or lines or the little neoprene jumper is/are leaking, and dribbling down the top of the K and carried away by the airflow, and then evaporating when the engine is shut off. The most likely is the jumper.
-I urge you not to drive it until you fix the problem. Id hate to read that your car burned up. 2 to 3 gallons in 1 hour is a huge leak.It shouldnt be hard to find.
-It might also be that there are multiple keaks. eg. the sender may not be sealed either.Pull the dipstick. Check for stinky, runny,excessively high oil level. If its in the oil it will be coming out the tailpipe, stinky, too.