Just in case anyone is interested in the original question Jesse.Bor on this thread has an interesting approach that doesn't require the use of a donor. Certainly not the classical approach but doesn't look too bad if you can stomach a chopped Riv. http://www.jalopyjournal.com/forum/showthread.php?t=422233&highlight=riv+chop
Google "Bill Mitchell chopped Riviera". Ryan's HAMB thread comes up. Beautiful custom just for him, Bill Mitchell that is. Love the slight chop. Hate what was done to the hood and scoops, the caps and tires....pathatic. The chop is perfect.
Wish I could share pics of the car I seen this week, but I was sworn to secrecy! But I will say the Riv I saw was chopped and highly customized and probably changed my view of these cars forever. Just beautifully done. As far as the roof goes i think it's all in what you do with the sail panels. That sets the tone for the entire car.