I have a 570 Street Avenger on my Mustang right now. I bought it remanufactured from Holley's eBay store a year and a half ago, and it was very lean out of the box. That car can't tolerate leanish mixtures anyway, but if I were going to spend that kind of money for a new one, I think I'd go with a standard Holley or even a Quick Fuel-based carb, like their new Brawler.
Remember that a 670 has the same venturi and bore sizes as a regular 600 Holley. The Street Avenger does have dual fuel feeds and a secondary metering block, so that may be an upgrade. And if you believe the internet, it's a crap shoot...some are fine, some seem really lean.
Good luck!
