I don't think the 450 Marlin is available in stainless.
If you want it because you think that the 450 Marlin is "hotter" than the 45/70, then you are mistaken, as the reverse is true. The limit here is the Marlin action, and since the 45/70 has 3-6 grs more capacity (4-8%) than the 450 Marlin, the 45/70 is good for 25-50 fps over the 450 Marlin at the same pressure. BTW, the 450 Marlin guys, when backed into a corner, claim that the 450 Marlin brass is stronger, but that's not true, and has been proven at well over the presure limit of the Marlin.
But if you want a 450 Marlin, you should go for it.