Since it's on the intranet doing it in IIS would be the only way to do it as far as I'm aware. If it was an internet facing site you could do it at the firewall level and leave the internal port what ever you wanted, just so long as the firewall knew what port it was.
Also using host headers can help if you have multiple web sites inside IIS.
But so you know, you can only change the port for the entire website in IIS. It's not possible to change the port for just an application or virtual directory under the root. So in other words you can change www.me.com to www.me.com:89. But not www.me.com/you/ to www.me.com:89/you/ and have it not affect your entire website in IIS.