That would be what I would look at doing. Feeding would be a puzzle to solve... the FAL and M14 both feed from one side and then the other, not the center.
Is there some reason you want a bigger mag in a bolt action rifle?
I believe that the Steyr scout has 10 round magazines available.
But as far as welding the bottom of another magazine to the Savage magazine, I'd be looking at G3 mags... I have a 40 rounder that is made from a couple of magazine bodies - they seem to hold up to a mig welder pretty well.
-tINY