Ben, I must advise against a custom .600 NE bolt action. First off, where are you going to find a bolt big enough for that giant rim of the .600 NE? Second, where are you going to find a gunsmith who can make the thing feed?

Better to spend $5K on a .600 Overkill. Your gun will feed and shoot great.

Or save up for a .600 NE double.

However, as person formerly known as an owner of a .600 NE, I would recommend something smaller.