For any car including Chrysler 300 timing belt is the most important part for gas grid. Nevertheless, you need optimal size for 3.5 motors installed in 300. For regular service buy Chrysler original belt which is produced specially for 300 3.5. The accessory should have 201 teeth and be of 30 mm width. Quite usual and ordinary belts were used in 2006 cars with engine code EGG.
Always buy toothed belt for EGG-E4 engine code with recommended width at 30 mm. For 3.5 motor service in Chrysler 300 you might buy lots of analog accessories. So, during servicing your 2007 Chrysler 300 purchase quality materials.