458 Win is right , of course; could be done in 375 with the right work in the right hands. I'm a 375H&H fan but would be inclined to leave that fine old action as is and go 9.3x62.
I must be getting to be a sentimental slob, but avoid hacking at things like fine old FN actions and original pre 64 M70's....both are rare enough to be scarce and I like them the way they came from the factory anymore. Plenty of other stuff out there from which to whittle a 375H&H if we want.
A 9.3 should be a piece of cake.
