Maybe going downhill, he may get that. My Dart in my photo gets around 14 on the highway. 17 would be a blessing for me. If you didn't know, all 340's were designed to be race motors, not grocery haulers. The 340 in my van gets 10 on a good day.
With the right carb, tune, and gears 20+ mpg is achievable with a 340. I've done it, steady highway cruising for 200 to 300 miles at a time. A van has such poor aerodynamics, like a pickup, it is very hard to get good mpg. High Performance itself does not mean poor mpg. Cars are set up for different purposes. If you have a drag racer with low gears, that is one choice and no, mpg will not be your thing. The 4.10 gears were only in the Barracuda a month or two. I could not stand them, plus I could not get any traction. My Barracuda was set up as a high speed cruiser/handling car, along with the good aerodynamics, 4 speed, and higher gears, 20+ mpg came as a bonus. My 383 automatic transmission Barracuda would get 17 to 18 mpg if I tried, or on a trip.