1. Figure out a different HDD mount and get rid of the vertical mount that's stock. That will free up room to...
2. Cut out as much grill as you can from front bottom leaving space to mount intake fan.
3. Install intake fan on lower front.
4. Tape the side vents closed
5. Cut out grill on rear exhaust
6. Replace stock exhaust fan with 120mm exhaust fan, mount fan outside case with 92mm to 120mm adapter like
this (or just drill your own mounting holes).
7. Run your fans on a fan controller so you can adjust for noise/airflow.
This would be the most efficient. Otherwise cut a mounting hole on the side and mount an intake fan, not exhaust.
-z