Cory,
I second the suggestion to buy and study the book "Building Double Rifles on Shotgun Actions" by W. Ellis Brown.The second edition is the most current.
I own and shoot two such shotgun based rifles and am very happy with them. Both were made by a follower of Ellis Brown and member of this forum. One is a Simson based .405 and the other a Beretta based .45-70. Their calibers are in the same neighborhood as the one that interests you.
Good luck with your project.