Chargers tend to have a little more room in the wheel wells vs the Challenger. I totally get what you mean about the 305-45-17 tire size. I have a set MT's myself in that size and hate how it looks still exposing that wheel gap and looking kinda "dinky'.
I'm going for the 315-50-17 size next and hope it will look much better and beefier in the rear as opposed to the 305-45-17 size. I know most including myself had to modify the rear wheel wells to fit our 305-35-20 tires so it shouldn't be a problem.
If you're regular 2.62 gear like me then we'd be faster on the 305-45 but I'll be modifying the car in the near future so I'll make use of a bigger tire like the 315-50-17 for the extra traction and possibly look much aestheticlly better in the rear filling up that wheel gap.