Man...I'm out of ideas.
Last time I checked, those 70-72 bumper guards were no longer available. Year One is looking for someone to start reproducing them again. I need a couple of sets myself for upcoming projects.
A few months ago, I altered a set of '74 bumper brackets to tuck in the rear bumper for a friend. Turned out pretty good, but wouldn't want to do that every day. Unless you're doing a resto, I would eliminate that filler panel and tuck the bumper in for a cleaner look.
Good luck, hope you figure it out.