Winchester's factory 180's are loaded with 69 grains of standard Win. 760 powder - interesting that any handloader can duplicate that charge. Their lighter 150 grain bullet loads have a special blend of ball powder that we cannot get. I expect that they are using Win. magnum primers.
Since the factory ammo is so accurate I would definitely start with 760.
Good luck with the WSM.
I have a 300WSM in Win Coyote Version. I rebedded the stock(Winchesters bedding was Hot Glue) and on my 3rd reload set punched a <.25 MOA group. Pretty impressed with my self and the WSM cartridge. But, this group was 66 grains of RL19 and 168 grain moly coated HPBT Match from Hornady, -10 mils off lands, not a hunting round. Haven't had a chance to work up a good hunting load, although the 150 grain ballistic silver tip from Winchester groups at .6 moa in my gun. good shooting.