Whitey,

Get a copy of this book before you begin:
BUILDING DOUBLE RIFLES ON SHOTGUN ACTIONS.
by Brown, W. Ellis.
Publisher Information:
Bunduki Publishing Ft. Collins, CO 2001

You may also be interested in this conversion story:
http://www.hotkey.net.au/~orrs/GreenerStory.htm

As some one who just went through this. I will give you some advice. Figure out all the costs in advance. Consider that most conversions have no provision for adjusting the poi. So you will be spending some time, perhaps alot, at the bench. Take a look at some of the guns that are currently on the market, eg, Pedersoli, FinnClassic, Chapuis to name a few. If your conversion comes anywhere near the cost of a new double rifle, then forget the conversion.

Pete

http://www.damascus-barrels.com/Movie.html