To answer your original question, an E-body Cuda is going to have a 2" shorter wheelbase then a Challenger. Also, if the quarters are still intact, a Cuda quarter has a horrizontal body line. The Challenger body line curves upward right after the door. The tail panels will be different. The fuel filler knecks are also in a different location. The Cuda filler kneck is behind the lisence plate. A Challengers is in the passengers side quarter panel. Finally, the Cuda has a more angled side window opening where a Challengers side window area is more curved. Even a stripped E-body should be easy to identify as a Cuda or Challenger.
I'm on the home stretch of restoring a very rusty 73 Challenger. I'd like to do an A-body someday if I have the money. I'll definately spend more up front to get a rust free car. Even if you replace all of the rusty panels, there is still rust damage that you will never be able to completely hide. Starting with a cleaner car will allways net a nicer car in the end.
Good luck in the hunt.