I'm not as familiar with 'Cudas but with the Challengers if you had a 340 and larger engine, you got the rear valance that had the cut outs. If this is applicable to your car, this MIGHT mean you had a /6 or 318 originally. Also I did remember a post a long ways back that said something about California cars, as 71340RT stated, having different exhaust.
I actually remember from Mopar Collector's Guide a 'Cuda came through with a /6! Keep in mind that probably isn't the original exhaust.
Who knows, unless you know the whole history of the car, a lot could happen in 35 years.