Need advice: 318 or 340 for a 68 Dart?

Also.... A 360 is slightly cheaper in the piston department if ether (318-360) need a rebuild.
A 360 has exactly the same HP ability as a 340 and does it at a lower rpm. (Street rides fellas, not race track engines)
The 360 produces more torque than ether of the two.
You can get good mileage with a 360.
As further food for thought, a 360 Magnum is readily available at a junk yard or in a inexpensive truck. The computer controlled fuel injection keeps the cylinders in very good shape. Cross hatch lines are still visible after a 100,000 miles.
Mileage differs 1 to 2 mpg's in mild to well built machines.

Think about it.