Why not just chamber a bolt action in .450 Marlin? A bolt action set up for the belted mag cases should work. One of the new Rugers in .350 Rem Mag would would be a good candidate. Thanks to the cowboy action shooters, there are lots of .458" cast bullets available for cheap shooting. Or, take the .450 Marlin case and neck it down to .429" if you want to go to the trouble.

-Bob F.