A lot depends on color. a H.O. car greatly increases it's value, whereas some colors, like G.R. on a 69, well, those cars are only good for washout/erosion bank filler?!
Seriously, this is very hard to determine because of the quality of the body/paint if it was reworked? is it an untouched, original paint? is it a 'frame off'?, is it a 20 footer(or more
)? Does the paint have noticeable flaws? were repro panels used? GM sheet metal? Is it a conglomeration of the past 30 years of repro parts and added options (some don't care if it is)?
Probably the only other 3 factors, to me, are: 1) Is it structurally sound? 2) is there solid proof the car is as you claim (docs)? 3) Does the car have curb appeal, like the right stance, look 'Killer'?
Without seeing, I will take a stab and say $35-40K