375 I cant fault your thoughts. I would look at buying a CZ550 in 416 Rigby and rebarrel if you are not nbuying a 450Rigby itself. That way the bolt face and magazine are set up for the case.

I like the 416 because you can drive 340 grin pills and 400 fast enough for 300mt shots. The 450Rigby I think is one of the few 458"s that could do the same with 458pills. I am not familiar with 458 bullets so can not comment on the ones you list.

The CZ 550 will give you the weight you want and will hold 4 in the mag.