Stock suspension challenger is likely limited to a 275 tire. If you search here, you can see some have stuffed 295 tires but you do not have a lot of gear and those tires will be a lot taller.

You need a stickier tire. I have no idea what options are available in France, nor what is legal. There are several popular drag radials, with the MTs and Hoosiers most commonly used, in 275/60/15. The MT drag radials are advertised for use on a daily driver but they are not really well suited for wet weather.

If you can get your 60' time down, the car should be able to hit at least mid-12s. You don't have a lot of converter, so slicks may not buy much over a drag radial. You will need to work on the simple suspension items such as shocks to optimize the combo.