The throating of your barrel plays an important role as it is the last part of the chamber the sees the bullet on its way. Get it wrong and accuracy suffers. Throating, of course can be done by hand, but the issue is getting it done accurately. When I get throats altered I have it done by a smith with the barrel removed and properly dialed in using a lathe. With leverguns I tend to work with the factory throat and work around it.
You don't mention the brand of rifle but if it's a Winchester or Browning made by Miroku, the ones I have measured are tight with a groove diameter of around 0.457. The tight barrel dimensions and no throat will impact on what bullets fit. A bore riding design should work well, ideally one with no front driving band or a very short one.
You don't mention whether you intend to cast your own or buy commercially cast bullets. In either case you need to try to acquire samples to check fit in your throat. I would suggest trying RCBS 45-400 FNGC. They worked in my Browning without altering the throat. Best to check first before buying a mould.
NOE make bullet sizers that allow you to size the body or nose separately. This may help if you have bullets that don't fit.
