If you're trying to get it to conform to the grille them just putting it on tight will do that. Other than that I can't really say. Mine's flush and I can't picture anything else.
^^^Thought about doing this ^^^
My plan as of now is to JBWeld or QuikSteel appropriately sized allthread to the interior of the grill surround, then attach the grille btwn w a lock nut. That way I can remove the mesh to service it if ever need be (ie: repaint, re-letter,etc.)
My concern with just having the mesh flex when tightening it down is that whatever I use to affix the allthread will be too stressed when trying to use as the anchor point for bending.
My other thought was to measure the overall length end to end, then measure depth from the center of the lower bar to the intersection of the furthest inside points of the outside edges of the grille. Once I have those #s, I can make a form to bend the mesh over. Will just need to find items to apply force with that wont pinch the mesh sheet. (I'll try to upload some "plans" once I can get some drawn up)
I don't think the radius is enough to need to account for a significant enough growth or reduction of size in the metal at the "bend epicenter" (for lack of a better term!)
What I MAY end up doing also is removing the curve in the interior side of the grille so the mesh will be flat across, but recessed enough behind the grille to not look out of place against any other remaining radius pieces.