You don't need to make a evox cd. Just burn a copy of slayer's autoinstaller (which can be found in the "usual places") and pop it in. So: put hdd back in xbox, boot to xenium bank with slayer's in dvd drive. it should load up and you'll find tons of options that will help - look in the readme that comes with it.
You'll likely have to reinstall everything (don't remember the exact options or whether it will completely wipe the hdd), but just take your time and pay attention to the slayer's readme.
Wait a sec, just reread your post: i think if you get slayer's up and running you should be able to ftp to the xbox. then you can just rename unleashx's xbe to evoxdash.xbe... or something like that. google for "upgrading from evox to unleashx". I know i followed a tut to do this, but its been a long time. think there might even be a tut here on xbscene.