The 428 cid Cobra Jet engine was the power source for 1969 GT-500, which lost its "King of the Road" markings. Though still rated at a nominal 335 hp power output (but closer to 400 hp in reality) it accelerated faster than the GT-500KR with a 0-60 mph in 6 seconds flat and a 0-100 mph in just under 14 seconds! On the other hand it took about 3 seconds more for the quarter mile and its maximum speed was rated lower, though that probably also was for insurance reasons (around 230 kph was estimated).
Due to the internal Mustang competition sales dropped and turned out to be disappointing. In 1969 a total of 3150 Shelby Mustangs were sold of which the GT-350 convertible was the least popular with only 194 cars sold.

This result led Ford to pull the plug on the Shelby Mustang project in 1970 and to concentrate its muscle car efforts on the Mach 1 and Boss Mustang models. The remaining 1969 Shelby Mustangs were renumbered and slightly retouched and sold as 1970 models.
The rare 1970 Shelby Mustangs can be identified by the two black stripes on the hood and a lower front chin spoiler. In other aspects they're identical to the 1969 models.
(picture by Charlie Ruggles, used with permission)

Only 636 Shelby Mustangs were sold as 1970 models, of which 350 were GT-350s and 286 were GT-500s.
The Shelby Mustang had gradually transformed from a highly successful race car, the first American race car to win an international race, to an overweight and underpowered production car posing as a race car. In 1970 Caroll Shelby turned away from the car production business and Lee Iacocca was promoted to president of the Ford Motor Company and so the paths of the two men who started the Mustang muscle car craze parted. However the paths of these two men were joined again in 1982 at Chrysler, where Iacocca asked Shelby to be the "performance consultant". This lead to a less exciting range of Shelby cars based on compact front-drive Chrysler products, like the Shelby GLH-S 5-door hatchback (based on a model known as Simca Horizon to Europeans)  or the Shelby CSX hatchback coupe. In the early 1990s Shelby inspired Chrysler to build the Dodge Viper RT/10, a car far more worthy of the Shelby name and kind of a successor to the classic Shelby Cobra sports cars.

In the 1980s Caroll Shelby constructed a dozen GT-350 convertibles from restored 1966 Mustangs, identical to the prototypes originally produced in 1966. They were easily sold for US$ 40,000 per car, proving the popularity and status of this range of cars.
Some 37 years after the last original Shelby Mustang model was produced Ford introduced a new Shelby Mustang: the 2007 Ford Shelby Cobra GT500.

Nowadays the Shelby Mustangs are popular classics, especially in the US where they hold sort of a super car status. Though production was relatively limited, many Shelby Mustangs have survived (or have been restored) and aren't rare to find. The early models are the most valuable, though there are also enough enthusiasts for the later models. Shelby Mustangs are still reasonably affordable, though prices for mint or rare examples can be up to ten times as high as the original US selling price.
In Europe Shelby Mustangs always have been exotic and relatively expensive. Popularity and acclaim of the cars is less than in the US but from very early on there has been a small but dedicated group of enthusiasts for these cars. Gradually an increasing number of Shelby Mustangs are showing up in Europe, usually recent US imports.
Because of the popularity of the Shelby Mustangs and the many contemporary standard Mustangs available there are a lot of Shelby Mustang "recreations" around: cars that more or less have been converted to Shelby specs and looks. Sometimes they're hard to distinguish from the original but more often they're hopeless pastiches of (copied) Shelby parts from different model years. I really recommend checking the history and VIN-number of the car if you want to buy a Shelby Mustang and to keep a firm distance from cars which look like a copy.

A fine expression of simple brute force these Shelby Mustangs, I hope you'll agree with me after this tour. An exponent from a time when youth emancipated and deviated from the paths laid out for them by former generations. As such a vital part of automotive history. But you might just as well like the car for what it is: undiluted driving pleasure...