Since you've already found the serial clock info in the data sheet, check and see both what the error is and which direction the error is on a 168 at 16MHz and a mega8 at 17.7MHz. If they both tend to be a few % high, maybe they'll match.
I would be tempted to just try it, and if it doesn't work I would dig into the information on changing arduino clock rates, get an extra 17.7MHz crystal, and run the arduino at 17.7MHz.
-j