Eudora doesn't have a provision for user-defined MIME charsets, it forces iso-8859-1 for all messages with 8-bit chars. You can report the problem to Eudora author Jeff Beckley and ask him to add user-defined 8bit MIME charset to future Eudora versions.
See also Win95 Eudora Fix.
P.S. I wouldn't recommend Eudora due to its Russian-hostile nature.