If it was me I would try the new.500/.416 NE. On a new rifle I too would get it scoped and ensure the rifle is regulated with open sights for th 400 gr loads and use the scope with a much lighter bullet - say 300 grs - even if only as a single shot. Hopefully both.

In a larger calibre I would go for the .500 (because I have a .450) or a .577 as long as the rifle wasn't too heavy and swung well. I don't see the need to go beyond .577 unless I went for a bore calibre and that would only be for fun.

If i didn't have a .450 I would choose the .470 (if I didn't want a .416) for all the obvious reasons.