I'm with cat, the .458x2" is a great little round. Brass could be made from anything too. The only advantage I see of the .450 is if you don't want to load your own.
The two rounds are almost identical with the .450 having a slightly wider belt and a very slightly longer case.
The 660 in .458x2" would be a great bush carbine, it would be very easy to sell to buy a .450 if you were set on that round.
With that said, I'm sure anyone with a .450 reamer could do this job very quickly and easily. In fact I think it could be done with a .45/70 reamer and bore the rim a little wider with a boring bar.
Nice custom rifle either way. I wouldn't be surprised to see a little .450 bolt carbine in production soon, it would be excellent for northern and bush work for the bolt gun fans. I know some have been made in the Remington and Winchester custom shops.