If you go with the 264 WM, it can be a love/hate relationship

But once you find the sweet spot they are a very accurate round with a 140gr. Unless you have a stock pile of H870, you can expect to be in the 3100 FPS range with a 26" barrel. H870 will put you over 3200 FPS & so far I have not found another powder that will duplicate that without getting pressure

I ended up dropping 2gr with my load down to 72gr, & cut the barrel down to 24.5". I am now in the 3100 FPS range with H870 & it is very accurate.
US869 works well, but you are in the 3050FPS range with that powder.
Out of the more modren extruded slow burning powders I have found RL25 to work well. Many 264 WM shooters recommend Ramshot & Retumbo. My rifle didn't like Retumbo & I never tried Ramshot.
50BMG may work well too, but I have never tried that either. Lyman lists a max load @ 70gr with a 140gr & 50BMG.
After all the headache I went through & am still going through (somewhat) with my 264 WM, if I had to do it again, I think I'd have built a 7mm Rem Mag for myself too. Or gave Dad the 264 WM & kept the 7mm I built for him myself
