Hi Stalker,
The 450Rigby CZ will be a nice rifle, should feed well as the 416Rigby CZ feeds great. I once owned a 458Lott on Brno 602 and found 2220fps with 500grainers was as much recoil as I could handle with the factory (458WM) profile barrel. At 2300 and 2350fps it kicked like a mule. Weight was just over 9 1/4lb, so if you were to make a 450Rigby I would suggest make with a heavier profile barrel to get weight up to about 10 1/4lb.
Cheers Graham.