You should first try it out with the standard serial connector. Get baudbandit or 8n1.device from Aminet. Should give you enough speed to run a 56k modem. You won't get a connect with 56k baud anyway. Usual speed is around 44k through a normal telephone line. If you get 48k you are lucky.
Note that there is a modem setting which selects whether the real connect speed is reported or the maximum speed. This is set to maximum speed by default, so most people just don't recognise that they actually don't get 56k.
