They have been converted to .416 Rigby ( overall length 3.750") but it seriously undercuts the lower locking lug and is not recommended. If you want a .416 the .416 Taylor (COL 3.340") will do nicely and you can get the same ballistics as the Rigby, it uses a necked .458WM case.
.375H+H is touchey ( COL 3.60") as well again because of case length but it has been done more often than .416 Rigby.
9.3 x 62 (COL 3.291) is not far from .375 in power terms and 9,3 x 64 (COL 3.370) equals or exceeds it and needs less done to the action. Both cartridges have enviable reputations for hunting efficency.

Both are good actions and there is not a lot to choose between them.

I personally would not put a scope on a .416, but I am not a fan of scopes anyway, I'd rather just get a bit closer.