No, it will not work, as 300 Win Mag is short action.
300 win mag is long action, 300 Winchester short mag is short action.
Is that a Typo? I would think he would need a long action Magnum stock for a 300 win mag...unless he a 300 win short mag. I re-stocked my 7mm Magnum into a Magpul stock that allowed for a larger barrel. My father's old ADL long action .270 fits fine but the factory shorter barrel has alot of extra room so it looks a little silly up close but it's still MOA or less at 100