A little confused...are you going with a "factory correct" style of tire/wheel replacement? Or are you looking for specifically the 15x7 combo for your ride? If you told him that you wanted what was on the car originally then I can see him wanting to steer you more for a 14x6 replacement, but if you specifically asked for a combo you wanted in a specific size why is he trying to sell you something different?
According to challengerspec.com 14x5.5(6) and 15x7 rallyes were both available and both optional. If you're talking about the Magnum 500 only the 14X5.5(6) were available.