I don't mean to disrespect what you are doing. I just see a lot of folks here recently concentrating on the front of the car.
If you are trying to decrease drag, the best place for that is at the back of the car.
There is plenty of information here on E.M. from user " Aerohead " on the subject.
Your car had attached airflow on the front of the car from day one. You have rounded the front end and possibly sent the airflow around the sides of the car. This may, or may not decrease drag. Yor modification has also plugged the grille and added a section that is lower - much the same as a spoiler, so this will certainly decrease drag and you will see MPG gains....but as you can see from the attached image, there is no need to extend the nose of the car. A simple grille block, and a front spoiler will do the same thing.
You asked about what to do at the back of the car. This guy did a wonderful job of taking a '3-box' design, such as yours, and streamlining it :
The only thing he might improve on is to round out the sharp edges on the rear extension.
Disclaimer : I am just an 'armchair' aero enthusiast. I just repeat things I have read. I am constantly surprised at new things that I learn about aerodynamics. I know nothing. Carry on .