Funny you should ask...
My son and I will be installing a 70-style Go-wing this weekend on his '74 Dart Sport. I got it for him last Christmas, but we haven't had time to mess with it until now. I bought it from Dante's Mopar Parts on the web.
When we started looking at how to install it, I determined that the wrong under-deck lid brackets and mounting instructions were included. They sent stuff for a '70 Dart (squared-off deck lid) not a Dart Sport (fastback style). The measurements were way off and the brackets were shaped wrong.
I have to say, if you have never seen the under-deck brackets in place, it won't make sense when you see them. For them to work correctly, you have to cut some sheet metal off the bottom skin of the deck lid (just enough for the bracket to recess). We got to see a 340 Duster with the same spoiler just a couple of weeks ago. Then it all became clear how it works.
I took a couple of pictures, but they are at my office. I'll try to post them Monday, unless someone else beats me to it.
Jerry