[WarrenG]

Like everyone else, I'd have to know what you're building before offering a specific opinion. That said, I agree, if you're building an FIA or Slab Side, I'd go with the 289. They can be easily stroked to 331 and provide almost 400 HP and still run on undoctored premium gas.
If you're building a 427 Cobra, you might consider the 428 it's just as original as a 427 SO and you should be able to get an old enough block to avoid emission issues (if that's even an issue in your case). It's also a lot less expensive than the 427 SO. Not as much HP as a 427 SO, but should be enough for anything except serious competitive racing. Personally, I like it better than the small block option for a 427 replica. Not only more original "looks" wise, but also possibly more original in the way of performance.