Need advice: 318 or 340 for a 68 Dart?

If your only choices are between the 318 and 340, and you want to spend less money, a running 318 would be my vote.

For reliability and mileage in a DD, I'd leave the S6 in there, frankly.

340 parts can get expensive. Lots of guys - me included - love the 340, but it's going to be more money, especially for the initial buy in.

I'll echo some of the suggestions for a Magnum 5.2 or 5.9, if you want to branch out a little.

http://www.magnumswap.com/