For the 375H&H I use 66gn AR2208 behind the 350gn Woodleigh or 350gn Taipan (I bought a couple of boxes of these at a gun show - not sure if they are still available, they looked like old stock) - velocity unknown but good accuracy. The ADI website doesn't list 350gn and the Woodleigh manual doesn't list AR2008 against this projectile weight.

For the 458WM I use 72gn AR2206H behind the 550gn Woodleigh RN or PP. No load on the ADI website and a couple of grains above the Woodleigh manual - velocity unknown but estimated 2070fps based on the Woodleigh manual, and good accuracy. Accuracy with the 500gn is better though, still using AR2206H.

Note that most of the loads in my 375H&H are generally 15-20% more than those on the ADI website - fine in my rifle and I have worked them up cautiously from recommended loadings. That's also the reason I use AR2208 - I can't fit enough AR2209 in it, even though many others favour AR2209 in this cartridge.