You can chop a boat tail at any point before reaching the tip. Compare AndrewJ's to basjoos'. Once you have passed roughly 50% of max. cross sectional area in the taper, you're into the realm of diminishing returns anyway.
As for constructing/mounting it, I don't think anyone can give you specifics. You'll need to either copy someone else's work (see above two cars), or get the creative juices flowing and come up with a new solution.
As for smoothness, either of Andrew's or basjoos' work could be made smoother. It all depends on how much time you want to put in the project. My experience is you can make something functional, or you can spend 10 times as much time to make it functional
and professional looking!
