JPK:
Just checked COTW, and it looks like the .450 NE reamer will clean up a .458 Win Mag chamber by 10 thou or so. If not, you could go for the .500/.450 Magnum NE or the .450 No.2. I know most posters here (including our host!) would favour the latter, but I like the lines of the former cartridge best myself.

Meanwhile, nothing wrong with modern .458 performance on game. If you could solve the loading/extraction problem for belted cases in the break-open gun, you would be an instant millionaire I reckon!

Good luck with it either way.