If you are ordering a new double, why not go for a .450-400 or a .470 or the short .577 Marrakai suggested, and while you're waiting for it to arrive, get a Ruger #1 in .458 Lott and some reloading dies...start shooting the .458 Lott with pipsqueak 1,000 fps loads in a .458 Win. case, and change loads every week to increase the velocity by 50 or 100 fps. Most likely if done that way you'll adapt easily and after a couple of months you should be able to handle just about any rifle made.

Richard.