Need advice: 318 or 340 for a 68 Dart?

The 318 can get pretty good gas mileage and can perform well. Both at the same time can leave a lot to be desired. Just a "growl" can be mufflers. Tell us what you would like from the engine as far as performance is concerned.

Pepping up a 318 to perform like a stock 340 will get you the same mileage more or less.