Well the only thing I've ever heard is that the Quadra Link is far better than even DSE's leafs. It's more adjustable and easier to tune. And I'm pretty sure it doesn't add any weight...after all it's only a few brackets and 5 links!
I have a AME parallel 4-bar and other than the 2x4 x-member that I added (DSE doesn't use one) the suspension itself, including the coilovers, is far lighter than the leaf sprung suspension.
If your budget and skills allow I would HIGHLY recommend going with the QL!
What front suspension do you have planned? You might consider DSE so the front is matched to the rear suspension.
I think the Chevelle you are talking about had
VBP on it...Mark Stielow had them on one of the red cars.