I have a 31x10.5W on my 66 charger.

If he is relocating I assume he is mini tubbing. He should be able to do the "W" tire. It looks nice in the minitub.

He should consider moving the rear end back and inch to an inch and a half as most of the fitment limitation will be in the diameter and the tire may hit the front of the rear wheel well. Use US CarTool hanger (advertiser here) that has multiple holes for locating the leaf spring front eye bushing. He may or may not need to move the rear hanger depending on how much angle it has before he moved the spring back.
Bob Spelic