ORIGINAL: whiplash
By rear valance i assume you mean that little black plastic strip on the bottom I would think that would be easy enough to either cut or order one for an RT and put it on since the bodys of all the cars are pretty much the same or am I wrong? From what I have read it seems that most of the differences in the car are engine based
I would think you could place the valence so you would then have 2 exhaust holes. I do not know if is plastic. On the older Challengers they were metal as plastic was not invented back then.
I have seen the list of SE vs RT vs SRT and it is a long list and more than just motor differences. ie. Brakes, suspension, transmission, wheels, electronics, etc.
There are even huge differences between the 5.7L and 6.1L.
I hope this helps.