to use NADA you got to have an 'original' car,correct engine etc.....since mine is not ,and many others arent , NADA cant be used for many cars,... you got to see what the "going market" is, by seeing what Collector car trader is asking , (using a +_75% of that as guestimate of selling price) and checking EBay to see what "comparable" cars sell for..it seems that 'non original' cars are going up in value too...in my estimate a non orig nice car (Firebird) is probably worth around 75% of an all original (in same condition)...
I figure mine (non orig engine, non 400 to start with, 5 yr old paint, modestly worn de luxe interior, etc) wuld be worth $18-20k...got it insured for $20k, I DO have $24k plus in her but hope I could sell for 20......as time goes bny ,maybe worth more? I consider mine a 10 footer...