Metalguy,
Welcome to the club! First off, I would suggest you buy the book "Building Double Rifles on Shotgun Actions" by W. Ellis Brown. It will give you the basics involved in doing a conversion. Next, you need to decide for yourself what kind of hunting you want to do with your new double rifle and that will indicate what calibers to consider. There is a list in the book of calibers that will work well in a conversion rifle.
For myself, and the kind of hunting I do, I have settled on a 45-70 as the most appropriate caliber for my own use. I use iron sights with it , mostly for deer hunting, but it will also handle hogs, black bears, or Moose, given the right bullets and the right loads. I also like the 45-70 because it is cheap and easy to reload for and is considered a low intensity cartridge in the loadings that I use in it. If you feel that you need something that shoots a little flater, there are several recommended calibers in the book.
Next, you will either need a lathe or have access to a machinist who has one, and is willing to do work for you. A milling machine is also nice to have but you can get by without it if you have to. Also, you need to understand that this will take many hours to do and no small amount of monitary investment. If you do decide to go ahead with this project, I wish you all the best and hope you have a successful completion of your gun. You have found the right place to ask your questions.

Bob H.