My father is into the original skyliner retractables (57-59) and he has a book with photos of the original mustang concept. It had the same rear end as a stock mustang, but the quarter panels were extended in order to increase the rear storage enough to accomodate the top. It also had chrome around the windshield posts like a standard convertible.
It was a much nicer looking car than these recreations, whose good looks are spoiled by the ugly rear window and clunky rear deck.
If I remember correctly, the book stated that the project was scrapped due to marketing hesitancy over the manual only top, and the original concept was destroyed. If anyone is interested I can scan the article and post the photos.
Here are the web photos of the original from the MustangsPlus website:
It is a much better looking car than the newer kits, as it still retains the mustangs classic lines. The newer kits are out of proportion in the roof and trunk areas, and are butt-ugly from the rear.