As far as the rim size, I would match what ever you have on the FRONT, same goes for the tire size.
Reason being, firstly with the tire height, if you have unevenly matched tire heights in front, it will cause the car to pull to the side with the shorter tire. It may actually pull fairly bad if the difference in tire height is different enough.
Secondly, also pertaining to the front, if you have unmatched rim sizes, in an emergency maneuver, it may affect the handling of the car as the sidewalls of the left and right tires will act differently. Its a small detail if you are going a short distance (And matters more with different sized rims front to back.) but might be one to consider depending how far you drive you car from home.
Tha newer 300's/Challenger/Chargers are a slightly different bolt pattern. The old E-bodies use a 4.5" or 114.3mm bolt pattern where as the newer cars have a 115mm bolt pattern which may stresss your studs when the lugnuts are tightened. It'll fit but may not be advisable for long durations of use.