You dont have to have a large shank for a WSM, although most likely all of the factory WSM barrels you find will be large shank.

Seems like I remember reading that they did make small shank WSM rifles for a year or two, but I could be wrong.

With a custom barrel it does not matter.

I think there is something about the firing pin assemble when going to the magnum, IE it should be changed.
You might want to ask SSS.