I went to the bodyshop today to trial fit the valance.
First, I bolted the valance in place, finger tight, and then drilled the filled-in upper mounting holes with a small drill and a Dremel tool, accessing the location through the headlight area. I made a pilot hole from the top side down through the mounting screw holes in the lower part of the bumper. Once I could see where the pilot hole was from underneath, I used a larger grinder bit to open up the hole to the correct size. Worked like a charm.
Once the valance was in place I held the front spoiler in the right spot. (the front fender flares actually do a good job of setting the spoiler in the right spot for alignment purposes.) I then used the Dremel tool to grind out the new holes in the valance using the spoiler's holes as a template. And as predicted, the majority of the original factory hole locations were about 3/4 of an inch off. So I was glad I filled them in. I drilled the new holes and used some nuts and bolts and set it all in place in a new position that doesn't require you to heave on and abnormally stress the valance when it's bolted into position.
The car's original white valance lined up beautifully, better than the blue valance. There's nothing like using the original parts from your car. (with a whole lotta massaging)
Here's the original before shot from October 2004, for comparison purposes (and to remind me of how this car is supposed to look when not in grey primer.)