'74 Duster Rear bumper trim

I am working on the rear bumper of my '74 Duster. I am having trouble with the gray plastic trim pieces that fill the gap between the body and the bumper. They have apparently shrank and now have a big gap under each tail light.

I have tried to hit the center piece with a heat gun and stretch it a little but, I don't know if I am going about this all wrong or not.

Has anyone else had a problem with these trim pieces before? I am not really sure what to do here.