Welcome to Shilen Rifles, Inc.
That link should have what your looking for, magnum being the #19 right there.
you can go the same as whats on it now, if the barrel channel is larger than the barrel that is on it now, you can get a bigger barrel, the same contour... if you want a smaller one, of course you will have that added gap between barrel and stock in your barrel channel...
looking at the mggowen sight... they can countour barrels to that factory magnum contour.
if you do want a larger barrel, and need to open the barrel channel up on your stock, a wooden dowel with sandpaper wrapped around it works great and ultra cheap to open up the channel a bit, as long as your keeping it straight it will stay nice and even.
hope this helps