Yes, the coupe front fillers will work. Instead of trying to replace one of the bumper guards, perhaps you can remove them and fill the holes while you're cutting and tucking the rear bumper...just a thought.
I've only seen one Regal wagon, and that was also in the junkyard. I removed some of the interior plastics (and tan cargo carpeting) which are now in my wagon.
Wished you had grabbed the bumper guard...I hadn't though of removing and filling the holes...I kinda like the look of the guards...adds to the Blue Hair theme i want to keep...I still have the 14 in wheels and wire wheel covers also...lol