For easy accuracy with cast bullets, there is hardly a better choice than a .45/70, or perhaps a .45 2.4" (.45/90)

With already having the .450 Nitro, you really don't need a high powered .458, ie; case longer than 2.1" - considering the .450 Nitro should shoot cast almost as well as a .45/70 - however the .358 Winchester has considerable merit for the lead bullets.

You could make a plinking,. cast bullet rifle as I did, using a shortened .45/70 case, ie: 1.9" for what amounts to a .45/60 Winchester, however using any bullet weight from 260gr. to 525gr. Mine, using a GM 18" bl. in a Sharps M74 action (re-shaped M68) is VERY accurate with both cast 506gr. and 525gr. as well as jacketed 405's @ 1,860fps. Of course in a strong action the 350 Speer could be pushed easily to 2,200fps if you wanted to, in that short .45 case.

So - my choice would be a 'held-back .45/70 reamer or a .358.

That 280gr. RN Lyman bullet is calling me in this venture, or one of the other makers of heavy bullets in that calibre. It would be a fun gun to shot with go-pow loads and with jacketed 200's or 225gr. Sierras, a terrific pig or Sambar rifle for out to 300yards or so.