I can't seem to find fuel pressure specs anywhere... Can anyone help? Key on engine running? Key on engine off?

 

Fuel pressure should be between 35-45psi on your Cobra.
Once the car turns on, the fuel pump should click on and pressurize the rails before the engine starts.

 

35 to 45psi with the key on engine off or at idle? Do you have a source for that info?

 

You should have that pressure with the key on and engine off as well as at idle. Don't have a source, just pulled it from memory. If you have a Hanes manual or some other DIY book for your model Mustang, it will undoubtedly be in there.