You'll be better off getting a custom .423" bullet mold. I'd recomend mountain molds, reasonably priced and excellent molds.
If you're starting off with hard cast .429" bullets, you're almost guranteed of breaking the handle on your lymann sizer going down to .423". If you are set on going that route, get a lee sizer and run them through your reloading press.
I'm not a gunwriter, that's 458 win. I'm just a gun-nut in Alaska.