Savage458,
I have done two of them, one for a customer and one for myself. The customer choose to use a Beretta 409 crossbolt gun for a donor gun. I choose to use an old German Sauer and Sohn crossbolt gun for a donor as it had bad barrels on it. Both were done on 12 gauge frames and both used Mc Gowen barrels, 1 in 14 twist rate. I'm not sure but they may be the only game in town when it comes to this caliber. I was concerned that the twist rate might be a little too fast because I wanted to use 300 grain bullets, but I found out that they shot just fine.
This was one of those choices that I thought I would never make, to do a .405 Winchester over a good old 45/70. But I was offered a chance to buy a partially done conversion in .405 with new barrels on it for a reasonable price. I thought at the time it would save me allot of work, but I was mistaken. I had to tear down the whole gun and start over because the barrels were not turned down enough the first time and one side had had the chamber cut too deeply.
It was lots of work to get it all straightened out but there was a good payoff at the end. I found that using the same 300 grain bullet weight that I had loaded in the 45/70, that the .405 shot flatter and held up its energy better than the 45/70. I was very pleasantly surprised. I think you will like the .405. If you want to see a few pictures of one of them, send me a P.M. with your email address in it and I'll send you a few pictures of one. Good Luck with your conversion.
Bob