The basic design was continued in the Blue Thunder manifolds, from what I have been able to find out. There were a couple of versions of the Shelby lettered manifold, depending on the carb bore layout. The manifold itself is about a half inch taller than a stock 351C-4V manifold.
That's the basics. I found one article comparing this manifold to others, and it received pretty significant kudos, even outperforming some of the single plane manifolds at the top end, while retaining good power down low. Its a pretty good manifold from everything I can tell.
How do I know? I purchased one for my 'vert, then found out about the height difference, which wouldn't work well with a shaker. Since then, I sold it, then figured out how to easily drop the motor the 3/8" that I needed. Dohh!
Value? Maybe $300-$600, depending on how excited of a buyer you can find, the carb bore layout, and the condition of the manifold. They are medium rare with the Shelby lettering, but since the same design is available in the Blue Thunder labelled manifolds, there isn't as much excitement about the lettering to the more performance minded.

Carl

 

I have one and would say around $300. I too got bit by the shaker/shelby combo.

 

Also, since a Shelby never came with a Cleveland, it would have been one of his aftermarket products, not much different than say, an edelbrock. Not a specific item "needed" for any model.

 

The blue thunder intake is highly regarded by many experienced Cleveland guys......Ive never read accolades for any other dual plane 4V intake higher than it.