You can, but some guys have had trouble with the drill bit deflecting when trying to drill into the existing slots. It's safer to drill just above the slots, deepen the slots to intersect the holes, and then trim off the bottom of the car but you said your car is already built so that may not be an option in your case. If you try to drill into the existing slots feed it slow with the drill spinning 2800-3000 rpm. Try a couple spare blocks first, check the alignment, and make sure you have the process down before you try it on your car.