Just open the client's preferences and change the port to 80. Just keep in mind:
1) If the admin catches you (very possible, even likely), you're the one getting burned, not us.
2) many trackers won't let you download a torrent through them if you use a port like that
3) and I just realized, I don't think there is a client that will let you set it to port 80, as that's reserved for http traffic.