Doug,
A GM color on a Cobra … well it's been done before. Shelby selected a GM color for his personal car. The last 289 (CSX2589) was repainted in the early 70's Corvette "Targa Blue" Metallic, with gold stripes. So looks like you are in good company
The PPG 5387 Navy blue is a great color. For my ERA, I went with PPG 4717 Indigo blue which is very similar shade of dark blue.