It's difficult to get a real headcount of clones out there, whether GT/CS. Shelby, Boss, or Mach 1. But, there is a good perception that it's happening more often.
It's so bad with the Shelbys, that Shelby, Inc., has really cracked down on you getting Shelby emblems. You have to prove that you actually own one to buy those emblems (which I doubt would really hinder cloning). The clone demand is why Unique is making those "continuation Shelbys". There is talk of them doing a coupe, too, but I haven't seen it yet.
Unfortunately, there are a lot of people out there that think nothing is wrong with making up a GT/CS. They think of those parts added is like adding A/C. They don't know that it ruins the marque, and that it's a REAL, separate type of Mustang; not just an optioned-out coupe. Between our efforts, the Registry, the upcoming book, and this site, we'll have an influence.
The lack of Shelby convertibles is simply the great expense of adding the roll bar. It's rare, and you have to find the Shelby type interior quarter panels, which are unique, and kind of box shaped. Branda once sold the roll bar, which was the same for '68-'70. But you'd be hard pressed to find one today.
I think what galls me the most are people's disrespect when they make a clone. Being an enthusiast is one thing, but passing it off as "real" is another. If you're going to modify your stock coupe, make either a Little Red, or Green Hornet, or just add the spoiler and end caps, and leave it at that. It's a vintage-modified Mustang. But to go farther and "fake it"as a CS, I feel is just wrong.
I recently had a friend in the registry just find out that their GT/CS that they'd had for years is really a modified '67 coupe. It's sad, because it's like the car had suddenly lost it's soul.
Paul N.
p.s. saw my first '07 Shelby on the road today.