670 is fine, I built a 340 (recently) that would run 12's with a 600 edelbrock, it is all in engine combo.
As far as the efficiency goes, edelbrocks and thermoterds are the way to go. The metering rods and springs monitor and fuel the engine off of vacuum, and not throttle setting. (for instance, lets say throttle is at 20% and you are travelling at 3000, then, you climb a hill, throttle is still at 20%, but rpm drops to 2800, the engine creates less vacuum, wich lifts the metering rods, and gives the engine more fuel, for more power) this also helps the carb run real lean at high vacuum, wich is how you get good MPG.
