Someone school me on 73 Duster rear bumper.

I wonder if the filler panel could be trimmed down to fill the gap?

Not really. I tried with one of the filler end pieces I had. The way they’re shaped and the plastic material they’re made of just doesn’t work well. I mean, you can do it, but it’s not as easy as it sounds and it’s not going to look very good. Or at least I couldn’t get mine to look good enough for it to be worthwhile. It doesn’t look bad without the filler, it just doesn’t look exactly like a 70-73 car. But you have to look right at that gap to know it’s a conversion, I drove my car for quite awhile with the earlier bumper and that recess just open before I added the metal flanges to mimic the earlier cars. The only place it’s different is in the bumper recess on the corners.