lathe spindle motor

I am in the process of converting an old atlas 6" lathe to cnc. The
spindle motor that is on it now is a 1/3 hp. ac motor. To be able to
use an encoder on the spindle (for threading) would it be wise to
switch to a dc motor? . I am very familiar with 4 axis cnc milling
but this lathe thing is throwing me.

Thanks,
Mike