I used mostly 270 and 300 grain bullets for the largest part of my shooting with my 375H&H and I have fired it so much in the last 44 years I had to re-barrel it because I shot out the throat on barrel #1.
The powder I found to give me the very best results overall was IMR4064.
The 10 grain lighter weight of your bullet would probably not make a lot of difference. Might try 4064 and see.
Yup, not a surprise. IMR 4064 has worked very well in many cartridges. I read that, at one time, the .375 H&H was factory loaded with IMR 4064 powder.
