Take this gun, make an entirely new set of barrels in 577NE or 12 bore rifled and keep the shotgun barrels intact.
I am not sure if it has bushed firing pins or not so maybe a light nitro a'la Fat Lady would be the best choice if not.
If I were serious about building a double rifle, I think that is the path I would take.

http://www.champlinarms.com/Default.aspx...&GunID=2479