3.23 or 3.55 Behind a 318
Like someone asked, what is the application?
For my street only application;
I've had 3.55s in my Dart for a year now. With 245 60 15 tires you can't floor it under 40ish mph without lighting them up. I love it but I want a better highway cruise. I'm going to switch to 3.23. A 408 doesn't need the lower gearing anyways.
I had a '73 318 Dart a few years back. Decent power with 3.91s and 14" wheels. Stout in town, useless on the highway.
A lot of this is personal preference... we're all different in what we like.