Need advice: 318 or 340 for a 68 Dart?

If it were me I'd go with a 318 for this build. Save the 340 for another build that's more performance oriented. For a daily driver a 318 would be a lot of fun and if you ever blew it up you could find a replacement for dirt cheap. Add in a 340 spec cam and good dual exhaust and I think it would suit your needs just fine.