Challenger and I assume Charger HC's use a 20 x 9.5" wheel, 18mm offset stock. With a 295/35 R20 stock. I posted a new thread today experimenting with an 18" wheel and 315 tire that seems like it will work. I was mainly concerned about the 18's clearing the rear brakes and several on here have said they did.
Look here: Anyone test fitting 18 wheels on the rear SRT Hellcat Forum
Look here: Anyone test fitting 18 wheels on the rear SRT Hellcat Forum