1973 Doge Dart Resto' Project

If that 318 doesn't smoke or knock, I would leave the shortblock alone and just upgrade the top end.
Find a set of "302" casting number heads, a cam that's close to the 68 340 Stick Shift one, an Edelbrock Performer manifold and a 600 CFM carb. Have someone who knows distributors set yours up so that you've got 36 degrees in by 1800 rpm or so. All that with a good exhaust...stock manifolds with either a 2.25 duals, or a 2.5 inch single and you'll get that musclecar rush and still get 20+ MPG.
I've got a lot of parts or that car, hit me up if you need anything.