GL-AR300M - Can WAN port be set to port 80?

I understand the reasons for having the WAN port to be port 83 from other posts (differentiate it from the LAN port and also since some ISPs block port 80), but is it possible to set the WAN port to be port 80?

Right now through the web interface I can change the WAN port to what seems like any port EXCEPT port 80. If I change it to port 81 I can see that /etc/lighttpd/conf.d/10-port.conf changes. But if I try through the web interface to then change to port 80 it doesn’t work and the config file doesn’t change. If I change the config file through ssh to port 80, when I reboot the router it changes back to the last setting that it was in the web interface.

Is it possible for the WAN port to be port 80?