From my brief run as a mac repair tech I can remember that virtually all macs made in that time era used the exact same modem card connection... the only difference between some of the modems were the angles at which the modem port would face relative to the connection. Some made a 90 degree turn, others didnt.
Even if they turn out to have different orientation (that link doesnt make it sound like it), it would be relatively simple to make an extension.