I see no easy fix short of switching those guards out for an OEM set and hope they fit better, or grinding on those until the profiles match up to the bumper.

Btw, I rebuilt an old Mustang coupe many years ago and bought repo parts for most of it. There was only a few parts that actually fit and looked like the original parts did. I had to slot holes and make alterations to things I did not want to to get the car back together. It was my first resto rodeo so I was naïve and assumed the parts I was buying from the big fancy catalog would be fine. Even the official licensed by Ford parts were not all that great to be honest. After doing that I learned why people pay so much more for OEM parts, or have their original parts restored instead of replacing them.