Got one of those too! Had to put it in a Remington with a Wyatt Centerfeed extended box. Just under 4" COAL with the 180g VLD. Much more FPS!
Much more powder too. About 65g with the WSM over 100g with the RUM.
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For a WSM in a SA you will need a WSM magazine, for a LA you can use a Standard Magnum Magazine (for a 7mm Rem Mag, 300 Win Mag, etc.).
If it were me, I'd go Long Action and then have no worries about your OAL. The WSM's I have had fed fine from Centerfeed Magnum Magazines in the Long Action. Also, if you build a WSM on a Short Action, you will need a WSM bolt head, or a bolt head converted from a LA-Magnum bolt head (unless of course your SA was a WSM to start with). For the Long Action you just use a Standard Magnum Bolthead.