Montana Native, I'm shooting a 300 WSM
(which is very close to the WM in ballistics) out of a 11 twist, 24 inch barrel and using 180 gr bullets. For the WM your most effiicent down range bullet, other than a wildcat will be probably be in the 180 - 200 gr range. My first choices would be E-Tips then AB's. That's just me.
For Montana shooting, with wide range of temps, I would go with one of the Hodgdon extreme powders, For a 300 WM, H1000, Retumbo or H4831sc might be good choices.