TCP port 443 is the standard TCP port that is used for website which use SSL. When you go to a website which uses the https at the beginning you are connecting to port 443. You should not use a different port number, because if you do then your users will need to enter the port number in the URL when accessing the Web SSL VPN.
You can redirect users to a non standard port without having them input the port number in the SSL VPN address by using firewall rules that direct traffic to the correct host and port.
IMHO there is no security benefit to using non-standard ports as port scans will discover any open and accessible ports.