Might just as well go for a 458 Win Mag or Lott as to build a 450M from scratch.
Case dimensions for 450M and 458 WM are really close, except obviously length. The limiting factor on 450M is going to be case length/capacity. And unless you're going to Africa after DG, even the 45-70 loaded up with some Beartooth Piledriver 525 gr or PD junior 425 gr at 1600-1700 fps will easily dispatch anything in North America. Getting to 1900 fps with something like a 525 doesn't improve ballistics significantly over 1625 or so, and it certainly doesn't improve lethality on the thin-skinned game we have here.
Plus, since Marlington doesn't make the 450M anymore, and they never sold a bunch of them, brass will be a lot harder to come by than either of the magnum 458s.
450M is a fine cartridge for what it was designed for, but it sure seems like you're trying to make a silk purse out of a sow's ear.