Need drivetrain input on a 68 Dart project

If you really want to clone the car then a 340 would be the obvious choice. You`ll have to change the drivetrain, K member, wheels, brakes, suspension, rear end, hood, etc.. There`s a lot involved when going from a /6 car to a GTS. If you`ve got the time and money, I`m sure you can find plenty of help here. Good luck!