Pac-Nor has listed on their website that they currently make .411 cal barrels, and also have available a reamer for the 400 H&H, you can order a pre-chambered, threaded, profiled barrel from them and just get your local gunsmith to headspace it and final fit the barrel for you.
Bet they will get a heap of jobs when word gets around, I'm sure an M70 or similar would be an easy re-barrel.
Take a look at www.pac-nor.com

450 Ackley