The .375 Weatherby is hard to beat. It'll do anything the Ruger will, fit in any rifle the H&H will, and still chamber and fire the H&H cartridge. That should take care of any imagined or real performance or availability issues.
The 375 WBY (or one of the 375 AI cartridges) makes good use of the obsolete 375 H&H case. It’s one of the few really sensible WBY cartridges. But WBY being WBY, they had to make the 378....
I was going to get a 375 AI built and even had a donor M70 action ready when the NEW KING was introduced - which was a much more practical solution.