Ford is reportedly working on a new track-focused Mustang that will serve as the spiritual successor to the S550-generation Shelby GT350.
According to information shared by Ford Authority, the upcoming S650 model will sit alongside the recently introduced Dark Horse SC, but take a rather different approach.

While the Dark Horse SC uses a supercharged 5.2-litre V8, the GT350 successor is expected to get a naturally aspirated V8, possibly displacing 5.2 or 5.4 litres. It is also expected to feature a manual gearbox, although the availability of an automatic option remains unclear.
In terms of performance, the new model is expected to slot between the standard Dark Horse, which produces 500 hp, and the Dark Horse SC with its 795 hp. Output of around 650 hp is a possibility, though Ford has not confirmed any figures.

The car is also expected to get wider bodywork, upgraded suspension, larger brakes and additional cooling capacity to handle regular track use.
Source: Ford Authority