Fill it in, the smooth look complements your car's stance and existing lines.
You do have a Cobra front bumper so if thats what you want then it matches/flows but after making such a killer ride it sucks to have someone call your car a Fobra.
If I were you and I had your bodywork skills I would ditch the Ford emblem-fill the holes, fill in the Mustang writing and finally show off the cubes you have under that hood with some updated badges.
Love your car, take it to the most expensive bodyshop in your area. Have them re-appraise it, fax the estimate to your insurance company, take all the money you can get from them then do your own work and put some of that money elsewhere.
Please post pics when its fixed!