I filled my front holes when restoring the car but then decided to reuse the front bumpers.
Here's what I did. Install all the brackets first. Then put some electrical tape on the bumpers where they bolt to the car. Loosly install the bumpers to the brackets. Get the two bumpers lined up and tighten the bolts. At this point the sides should be setting against the fenders. If everything is the same on both sides and the bumpers are level across the front, mark your holes, remove the bumpers and drill the holes. Make sure you install the inner brackets for reference as well.
Basically you're installing the bumpers before drilling the holes. Get them where you want them, mark them, remove them and drill away. If you're worried about your accuracy, drill a small pilot hole and reinstall the bumpers checking against the pilot hole.
Good luck.