Congrats on your rifle. It sure looks like a keeper.
As for bullets, the Woodleigh .408 will work just fine.
Accurate and damned nice killers. At least they were in
mine.
Yes, you can size down the .450-3 1/4 but a real PIA. I
did it, ended up having to buy a reamer when the necks
ended up too thick after sizing. Look around and find
some formed cases. It will be worth the trouble. If you
get the rifle and you're not all set, PM me. I'll loan
you the cases, dies etc. you need till you are up and
running. You can't be that far away.
Good luck
Raff