It really depends on the engine, but typically at idle a carb might run smoothest a little on the rich side, say 13.5-14. I imagine your engine being a 408 stroker probably means it's no slouch either, so really whatever ratio makes it idle best is what to use. I have my 5.7 running around the mid 13's for idle ratio now and it seems pretty happy. May be able to lean it up some now that I figured out some computer issues, but I haven't messed with it. You can also tune by trying to get the highest vacuum possible at idle and see what ratio that gives you.
For cruise it's really just as lean as you want to push it without pinging. I have mine set up to be in the 14's on cruise, 14.6 at my main cruising rpm. I can maybe even push it to the lean side a little more (I've heard some people push engines to 15 or so), but I could feel the engine surge and lose some power when it was dipping in the lean side on the inital tune when cruising. You also have to watch the timing when you go lean. Typically you need to add some timing since a lean mixture burns slower.
That's a bit of a misconception a lot of people have. An engine burning lean does NOT burn hotter necessarily. If you put less fuel in, you don't burn as much, so you can't produce as much heat, it's thermodynamics. Sure you get some air charge cooling from vaporizing the gas, but I highly doubt you'll lower temperatures 100 degrees by vaporizing gas. You'll cook exhaust valves because the lean mixture burns slower and burns into the exhaust stroke instead of using up its energy during the power stroke.
