You don't need any of the required hours to do the exam so, if you can, do the exam before the cut-off date to take advantage of the grandfather rules for the soon-to-be-old way to get an FAA ATP.
Speaking - or writing - of the old way, I wonder if employment by an air carrier (135 or 121) will continue to keep the exam credit beyond the two year period, as it does now? And, if that continues, do candidates remain eligible to use the grandfather rules to get their ATP under the old system after the two year period, or must they join the new 10hr sim trained method?