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.
I have a ruger 77 custom in 450 Marlin. Since it is on a bolt action and I don't have to worry about a levergun pressure requirements I started with the max loads on the Hogdon web site and worked slowly up. You can also look at 458 American loads. The 458 Armeican is a 2 in case if my memory serves me and the 450 marlin is 2.1 in. They have about the same case capacity. I will have to look at my notes for the loads I found that gave best accuracy and what the speed was with them.
It is a fun short action thumper. I say if it floats your boat go for it. I enjoy mine
Mule deer and Colorado elk seasons almost done! Hunters driving farm roads, looking for racks, their PH driving them along, I ask that you not pull into my drive. The buck behind me, on the boundary line of the GMU somehow knows. The hunter laughs, I would invite you in to see my Searcy rifles but social distancing prevails, darkness arrives and the buck slides away into secret tree grove...