Crazy idea. Get a normal modem.
There is no such thing as a 64 bit modem and there will not be either. No one is going to make a more than an eight bit chip uart. and the pretence by connexant that they will make a driver for their passive chips is a market ploy to keep using your code for their cheap chips.
Why use your processors code and time to make a cheap peripherial to work for the few pennys difference in the chips.
Spend your $20 on a hardware modem.
Please note I am using the modem as intended and no connection with the so called adsl modem which is a misnomer I believe that device is a TA and not a modem.