I have formed the 30-30AI from many different cases. I found no real value in the effort required to form the cases from the 375 Winchester. In spite of what it would appear you are not handicapped by fire forming your cases from NEW 30-30 cases and then loading them as higher pressure Improved loads.
Here is a link to a discussion on the Beartooth board. Assasin is gunsmith David White from D&T custom guns. David has a foum on his site too and a lot of 30-30AI stuff.
On the Beartooth Shooters forum use the advanced search with .30-30AI and William iorg and you will find many discussions of the 30-30AI and brass.
I like the 30-30AI cartridge but if I were buyinhg a new rifle today it would be a 308 Marlin Express as it pretty well renders the 30-30AI obsolete.
I do not recommend using used 30-30 brass for forming Improved cartridges, particularily if you intend to load to high pressure.
http://www.shootersforum.com/handloading-procedures-practices/38878-30-30ai-cases.htmlBroken cases in post 6
http://www.shootersforum.com/wildcat-cartridges/22963-need-hints-advise-fire-forming-cases.html