1989 Cadillac Brougham rear bumper fillers
 

So i replaced my bumper fillers to fiberglass and now it seems like my bumper dont mount like it should, any suggestions would be great.

I was told to use a drimal to shave the lip down

Bumper fillers are not fitting like the original ones did, the aftermarket set are fiberglass, should I sand and grind on them?