If I was intending to scope a .470, I would prefer to use claw mounts over anything else.
Pivot mounts are plenty fast enough but that is not the only issue.
If you know someone who can fit claw mounts correctly, that would be the way to go IMO.
Here in Aus, I really don't know any gunsmith whom I would be confident could do the job the way it really needs to be done.
If your double already has the rib milled drilled and threaded for bases you are part of the way there, and from all reports you have capable gunsmiths/makers who can do the job right.

What I'm trying to say is that I would PREFER claw mounts, but would not feel handicapped if I had to use a quality pivot mount system.