I would not bother with the .458X2/.458 American. You have a ton of money and time invested in shotening and converting brass for the new round. In addition, there is still the possibility of chambering and firing a short cartridge in any of the belted magnum chambers with disasterous results which is why the .450 Marlin came into being. ER Shaw and most custom barrel makers chamber for the .450 so the cost would be acceptable. You have a ton of loading options which make this a .450 a very versital round.
Terrier