you seem rather fixed on the idea of getting an american standard neck. Let me tell you now that the two builders listed above (
warmoth and
USACG) make necks that are comparable in quality. Some really great custom builders use these necks for their guitars. And honestly, nobody in the audience is going to say "wow, that guitar would sound SO much better with a real fender neck!" It's your call, but if you want lefty without having to replace the nut... one of these sites is probably the best for you.