I'd start with new brass, 30-30's are pretty cheap and easy to find. Seems like (to me) that new brass forms easier because the inside that has to drag over the expander is smooth and clean. Just a suggestion.
When you form brass for the 357 Herret, you have to work the shoulder back a little at a time. The 357 Herret actually headspaces off the shoulder! You want to have the barrel handy, push the shoulder back until the barrel will just snap shut with a little force. That ensures that the shoulder is as snug as possible.
Also, you'll need to have your case trimmer handy. At some point while working the shoulder down you'll have to trim some off the case to get it to fit into the chamber.
You size a little, then trim a little until you get a case that fits snugly. Then trim to the proper length. I know this sounds like a pain, but it's only on the first case that you must do this so carefully. Once you have set the forming die for your barrel, and have the case trimmer set, you can get after it.