If this is a newer stevens, it should be a centerfeed. Is the mag box connected to the action, or contacted to the stock? Staggered feed has the box connected to the action. I don't have a centerfeed mag box here to measure, but if someone can measure there's for you, it will let you know if it was a mistake or a design feature? Could be someone placed the wrong box in the stock? My guess is that you have a small shank action? If so, it will be hard to find a factory 300 WSM
barrel as they make them all large shank in that caliber. You could sell the 300 WM barrel and apply that to a custom barrel. The 300 WSM is sure not a bad choice.