Thanks guys. I also posted over on a trifive forum and the range was $8k to $14k with the average being $10-12k.
I've talked to him a little since but we have not struck a deal. He seems intent on the car going to me, probably because he wants to watch the build and go for a ride when I'm done.

I'll get a little more serious in a few months when hopefully the GTO is on the road. I've still got a 70 Z28 project sitting in the shop that I was planning to start next so this would throw a bit of a monkey wrench into that plan.
Don