K80:
My advice would simply be to shoot the heck outa that thing and enjoy it! You have just acquired one of the most sensible, practical, and versatile chamberings for a big-game bolt rifle, ever!

I load mine to modest ballistics with Woodleigh Weldcore bullets for big game, 410gr 'Rigby' bullet at 2280 fps with Aussie powders, but 2400 fps at safe pressure is a snap, especially with the range of powders you have available in the US. I have killed many buffalo, including some very solid bulls, with the 2280fps load. A Woodleigh soft will exit if it misses the shoulder-bones, or pull up under the off-side hide if it smashes a shoulder on the way through. Marvellous stuff!

Remember that the .416 Rigby made its reputation as an elephant rifle with 410gr RN bullets at 2350 or 2380 fps, depending on which catalogue you believe.

Here's a photo of the last bull taken with my Taylor, in the hands of my daughter's friend a few months ago:

One thing: I neck-up .338 Win Mag cases, rather than necking-down .458s. This results in a slightly thinner neck which is less prone to work-harden and crack. Also, I hear that .458 cases can be hard to find occasionally over there, and .338 cases are generally cheaper anyway. To neck-up .338s, you will need a long tapered neck-expander: mine is a custom die by Peter Davern, but I believe (from other posts) that the Lee die has a suitable tapered neck-expander. I have done a couple of hundred .338s, zap them straight up in one pass, never lost a single case.