For clarification:
The '68 Mustang GT-500 convertable IS a muscle car and worth up to $100,000. There were less than 5000 made. And it has a 428 Cobra Jet engine. The Shelby Cobras or A/C Cobras origionally came with 289 ci engines. However the Ford 427 was often seen in them. The Cobra Jet was a 428 NOT 427. Ideally, one would have a 426 Hemi Cuda, a 427 '67 Vette convertable, and a 428 '68 Mustang Cobra. That gives you the best big blocks from the big three. However I might substitute the Vette for a 427 Shelby Cobra.