Butch Smith @ Smith Automotive Machine in Frankfort, KY has done some machining on duramax engines for some truck puller buddies of mine. He has been building racing engines for years and has a good reputation in the gasser world.
Skytower in Stanford, KY has done a ton of diesels so I know he has the proper weights and equipment to handle them. I can't say for sure if he's done duramax engines but if I had to guess I'd say so. I know he's done a ton of Cummins engines. You could call and ask him.
Kenny Pulling Parts & Machine in Stanford, KY has big involvement in diesels. He mostly does stuff with Cummins and IH's but I know he's done some stuff with the duramax too. It'd be worth a phone call because even if he doesn't want to do the machine work for you, I'd trust a recommendation from him because he's a good, honest guy.
You could try getting in touch with Ryan Uptegraft @ Uptemax Diesel in Carlisle, KY. He doesn't do machine work himself (at least he didn't the last I knew) but may be willing to share who he uses for machine work. I know they used to use Burkett Racing Engines but I believe that fella died so they may use someone else now. Whoever Ryan uses will be well suited for doing the work because Ryan has built a bunch of duramax engines.
Obviously Wagler will have quite a bit more experience with the duramax than any of these I've listed.