You have two choices from what I can see:
Re-drill the lower mounting holes so the tabs pick up the new holes a little further forwards. You may also have to slot the holes in the slam slam panel to allow it to move forwards a few mm and shim out the two side plugs.
You can just see the shims on the upper/outer mounts in that picture and that the slam panel has had the holes slotted to allow it to move forwards slightly.
That got me a grille that would sit flush with the unpainted line.
Or
Paint the bumper to match the grille shape.
Option two would be the neater solution.