"I was talking to the guy at the local stereo shop"
That's cool, and I did the same thing when I was awaiting my empeg. However, my "guy" seems more knowledgeable than your "guy"
Unless he knows something specific about the Viper and has worked on a few before, he should know that the empeg will fit. Tell him it's a standard DIN and then see if he tells you whether or not the "pull-out" will fit. However, as someone said, dodge may have a wierd setup in there.
I think the advantage I had was that the guy I was dealing with had already done an empeg install before (at the time, August 2000, this was extremely rare), so he probably knew that the empeg was of standard size.