I'm not familiar with the lathe but, assuming it has an Imperial leadscrew, it should be easy to puzzle out the required ratio.
Say the leadscrew has a pitch of N tpi. One revolution of the LS will move the carriage 1/N inches. To cut a 6 tpi thread, you need to have it move 1/6 of an inch for each revolution of the spindle.
Thus the spindle to leadscrew ratio needs to be N/6. Any set of gears that yields this ratio should do the trick.