1. Three seventy-five Winchester brass is stronger than 30/30 Winchester brass and is probably the best choice for making 357 Herrett brass. If loads were worked up with 30/30 brass, a load adjustment for 375 brass will probably be needed.
2. You don't really have to fire form even 30/30 brass. You run it into your 357 Herrett full length sizing die, then into a trim die and cut off most of the excess with a hacksaw and then file to length and shoot.