The guy at the local car lot is the guy who told you it was a GM corvette...
You may have better luck if'n there's an old Buick dealership nearby. From time to time, you'll run into (figuratively, mind you) an old-timer that remembers them new - if he's part of the dealership, he'll appraise it at pretty much whatever YOU think it's worth.
If not, do the following:
a) Tally up the bill stack.
b) Estimate the # of hours in the car.
c) Organize the photo journal.
d) Check off some of the pricier GT's on Ebay - put them in your 'watch' column. Closing values are important.
e) Go to http://www.traderonline.com
and do a GT search under collector car trader. There are always a few higher dollar cars there - put them in your records also.
Armed with this stuff, you'll be able to provide enough documentation for even the most dimwitted used car geek to arrive at an intelligent appraisal. You know your car best - it's up to you to make sure they understand this!