Does anyone have any experience or opinions on the Mauser 98 in 375 H&H ?

I understand that the modifications needed to get the H&H to fit in the standard length 98 weakens the action. I think you would be better off with a standard length case.

Have a Interarms Whitworth .375 H&H and have had no problems with it.

Lots of fine rifle making houses having been doing this for over 100 years. Alternatively, there is the M70 which is already long enough.