The 458 Win was designed to duplicate the 450 Nitro Express in a standard length action bolt gun. The 404's original load was designed to duplicarte the 450-400 Nitro Express. The 416 Rigby was designed to one-up Jeffrey with insignificantly less speed and power. The 416 Ruger slightly beats them both in a standard length action. For ease of conversion, and Factory load availability, I vote 458 Win and 416 Ruger. Duane Wiebe makes nice bottom metals for both, and he can point you in the right direction with feed work:

http://www.customgunandrifle.com/products/bottommetal/mauser-bottommetal

Matt