Any suggestions will be appreciated.
Not that they're nice suggesions:
1) don't use Stunnel for something as already broken as ftp. If you must use ftp w/ ssl support, get an ssl enabled ftp server, such as proftpd.
2) ssl-protected ftp is not going to work through firewalls nicely because the firewall can't inspect the packets to re-write the IP addresses, or allow the ephemeral ports, so you're stuck hoping they have a permissive outbound firewall, you've got dedicated ports open on your firewall that allow anything from anyone without restriction and your ftp server is hard coded to use only ephemeral ports in that range, and that the client uses passive ftp only because active just plain won't work.
3) ftp icky. icky icky icky icky.