For your proposed .470, or for a .450 NE, (or better yet, a .450 #2 for better lower pressure) or a .500/.416 Rigby (for a lower pressure .416 Rigby bullet), and numerous other cartridges too (not .500NE or larger, as they are too light in weight), any Good condition, tight bolting German action OF 12 GAUGE, with Greener Crossbolt, especially one made from 1950-1990, should work just fine---they did for me! If you can find one, one with 3" shotgun chamber, that was intended as a heavy waterfowl gun, will usually give a bit more weight for better finished balance between barrels and action/stock. I took class from W. Ellis Brown at gunsmithing school, prior to his writing book on subject, now in 2nd edition, and I just follow his methods, with a few of my own modifications in building doubles. Get the book and read it. I know a guy who built a .470 too, on converted Ruger 12 ga. O/U successfully too, something I found interesting.