I believe the best way to keep attacking the Zodiac’s 340-character cipher is to create more test ciphers that share many of its features. If we can easily crack test ciphers under many different schemes, and the Zodiac 340 still won’t break under the same attacks, then the schemes can be systematically eliminated.
I collected about 200 test ciphers to compare with the 340, and posted a detailed summary of my approach here:
Long story short: Many people have created many test ciphers, and in most cases they share some but not all of the features of Zodiac’s 340. I’m hoping to help close that gap so we can at least start with a really good set of test ciphers.