may be to late to help but my "handloaders manual of cartridge conversions says. "make from 375 win. anneal case cut to length and chamfer, F/L size.....use .320-.321 dia. bullet
375 cases are cheap
32-40 brass is available on some of the online sites,however bullets with the crimping caulare in the correct spot are a custom deal. Most shooters who are not into purity use 30-30 brass and commonly available .32 cal bullets from hornaday or speer.the 30-30 case is shorter than the 32-40 case but with 32 special bullets oal. ends up allowing you to crimp the bullet at the canulare and gives near perfect functional length.