AvidFlyer, you are very lucky to have this Springer as your first double rifle!
Not only is Springer one of the best Continental makers (in my humble opinion) but once you get her shooting, I'm sure you will be rewarded for your efforts.

As Mike says, starting with 450/400 3" brass may be the most convenient answer to the brass question but some inside neck reaming may be needed to accommodate a .458ish bullet as the wall thickness of the parent case trimmed that far down may be thicker than a "normal" neck would be.

If you have only 450NE (which these days is much harder to find than the 400 brass) you could modify the rim by "waving" the rim.
I think I may have just made that term up...
Anyway, one can wave or dimple the thin rim of a cartridge to accomplish the task of keeping it against the breech. Think of a wave washer.
I've seen it done, but fortunately haven't needed to resort to that method myself.

If you need a handfull of 400 cases, I can help you out.
Or, if you think that this is more headache than you want, and want to pass this Springer on, I can help you out with that also...LOL...

As usual, Kuduae has cleared the fog, so to speak.
Obviously, you have found the right forum. It's all downhill from here...