There are I think two aspects to consider here. As Hack has said reducing the cast will tend to make the gun point more to the left. On the other hand changing the pivot point (shoulder contact) to the left will tend to make the gun shift more to the right in firing. Which of the two will hav the greater effect is hard to say & in either case will affect both bbls.
I am assuming though the owner thinks he can live with the existing seperation, just wants to have the "Center" of the two impact zones on aim rather than to the left side.
