@manx69, as you become more familiar, with your Classic Roadster. You will find what a different kind of Cobra you have. And I'm not talking a bad kind of Cobra, far from that. Just different. As you know these are just recreation's of what a Cobra was. So there is no wrong answer building your Cobra. Just your idea of what you want. Our Classic Roadster's have 6 inch longer wheelbase, so right off the bat, any Cobra person worth their salt, will never mistake it for a real one
Now to get back to your question, you may not find a serial# anywhere on the chassis, or body, mine and many more never had them. Hopefully you have a "Certificate of Origin" to be able to title and Lic. the car. As far as the Sebring & Cobra having the same chassis, someone else may be able to answer that. I have many Cobra friend's that have nicer "Some with more money invested" then I have in my Classic Roadster, but I love mine, as much as they love theirs. This club, is here to help anyone, that care about their car, no one here will ever look down at a lesser Cobra. Just do the best you can, ask any & all question's, we are here to help. Some of us need more help then others
Cheers Tom.