Yes you can nominate the port you want to use and that is the correct way to do it. If you want mIRC to connect to a server that is available just use the network's "round robin" if they use one. This means that if you try 'irc.efnet.net:6667' you will connect to the first available EFNet server that allows that port, but providing this is a round robin and not just a direct link to one server. I haven't used EFNet for a long time so you may want to confirm this in their #Help room.
