Stop right there. If it is 78 and 79, you have PC Card mail, which CAN'T be expanded in the number of ports, only in the number of mailboxes. And that is only by replacing Partner Voice Messaging PC Card "SMALL" with "LARGE"
Now, if you have Partner Mail VS, a module in the 5-slot carrier, and are replacing the PC card in that unit, it is NOT 78 and 79, so you are going to have to step through #505-7 until you find the extensions that are already assigned, and add the 2 prior to those.