Hi ThunderChaw,
Taking your comments at face value, yes the car sounds absolutely worth $5k. But your description of "runs great" and "great condition" can mean so many things to so many people. Not to detract from your observations, but what I mean is, a body specialist who sees bodywork everyday might consider your vehicle "average", so this perseon might not think it is worth it.
Value is in the eye of the beholder, and $5k isn't a lot for any kind of car, let alone a good condition classic T'bird.
My advise: If you need certainty about the value, continue looking online for other 58, 59, 60 T'birds for sale and compare the asking prices and vehicle conditions to yours. Look on eBay, Cars-on-line, etc, etc.
Go to
www.google.com and type in "58 Thunderbird Sale" and see what comes up.
Good luck and happy motoring!
- Art